What would happen if a wizard calls a familiar in the Ethereal Plane and then returns to the Material Plane?

What would happen if a wizard summons a familiar (using find familiar) in the Border Ethereal, and then the wizard returns to the Material Plane (interrupting or ending the magical effects of etherealness)?

Would the familiar disappear due to the distance from the caster, since they are now on different planes?
Or
Does the Ethereal Plane overlap with the Material Plane (Border Ethereal) such that distances are preserved and the familiar remains in the Border Ethereal while the caster is in the Material Plane?

The Ethereal Plane
Its shores, called the Border Ethereal, overlap the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane. (PHB, p. 301)

Does this make possible the ability of having one’s familiar become an ethereal traveler?

Will loot you find on the astral plane while using astral projection remain when you leave it?

While under the effect of astral projection you have an astral body which has the following rules:

Your astral body resembles your mortal form in almost every way, replicating your game statistics and possessions.

I don’t think that you keep stuff if you pick it normally because the following lines (but please correct me if I’m wrong).

Any damage or other effects that apply to it have no effect on your physical body, nor do they persist when you return to it.

It doesn’t specifically say that you cannot pick up loot and take it with you, but it differentiates the astral form from the physical form in a way that they are in no way connected (except the cord).

But, my possessions are being replicated, also my bag of holding. If I pick something up in the astral plane and put it into my bag of holding, will it remain in there when I return to my physical body?

Can a ghost possess someone in the material plane while in the ethereal plane?

The possession ability of a ghost (DnD 5e) states that it can possess one humaniod it can see within 5ft.

I interpreted that a ghost could use its possession ability from the etherial plane itself, since the ghost is visible on the material when in the etherial plane and as such can see the creature. Like this it is possible for the ghost to come up to a humanoid without being able to be hit and possess the creature unhindered.

If the ghost gets shunted out I rule that it ends up in the material plane as the ghost has to enter the body of the creature (who is in the material) while possessing.

However, one of my players mentioned that it is an action for a ghost to change from the material to the etherial and vice versa. He then argued that the ghost should enter the material plane first as an action first before having to spend another action on the next turn to attempt to possess a humanoid.

I can’t really find anything specific that tells that one or the other is true. Obviously the former method is more powerful and while I don’t want to be unfair to my players, I think it is also a whole lot more interesting.

Has the Underdark ever been a separate plane to the Material Plane?

I play D&D 5e; I am not that familiar with the other editions of D&D. However, I’m looking for lore on the Underdark from any edition, since the settings (e.g. the Forgotten Realms) are still roughly common to most editions (even if certain events have occurred in some editions and not in others).

For context, in my own homebrew universe, I’ve decided that the Underdark is in fact another plane, although it is accessible from the Material Plane via certain tunnels and such that are like subtle portals (similar to Fey Crossings). However, this question is not about my homebrew universe (which I doubt I’ll change regardless of the outcome of this question).

I was looking into the Underdark, searching through information online and in 5e books, to see if the Underdark is a different plane or whether it is simply beneath the “surface” of the Material Plane. It seems as though it’s the latter, which means I’ll have to go to greater efforts to adapt existing adventures to my homebrew universe that were written in the Forgotten Realms, for example.

However, I believe I got my idea about the Underdark being a different plane from somewhere, so I was wondering if there have ever been any adventures or settings within D&D where the Underdark has been considered a different plane.

I’ve read online that Matt Colville has used this idea from an adventure called “Night Below”, which was apparently an old 2e adventure. I did used to watch some of his videos, so maybe that’s where I got this idea from? But even if this lead proves false, are there any adventures or settings that have ever treated the Underdark as a different plane from the Material Plane?

Can a Divination wizard in the Border Ethereal use the Portent feature to influence rolls of creatures on the Material Plane?

A particular School of Divination wizard is native to the Ethereal Plane (he was created by a wish spell replicating simulacrum cast on that plane).

While this wizard is in the Border Ethereal, can he use his Portent feature to affect the rolls of creatures on the Material Plane?

Will the Astral Projection spell end upon entering an Outer Plane?

The astral projection spell states:

If you enter a new plane or return to the plane you were on when casting this spell, your body and possessions are transported along the silver cord, allowing you to re-enter your body as you enter the new plane. Your astral form is a separate incarnation. Any damage or other effects that apply to it have no effect on your physical body, nor do they persist when you return to it.

The highlighted text seems to suggest that, when you enter a new plane via astral projection, your body is transported there as well and you may re-enter your body. This suggests you have lost your astral form and are now physical.

However, DMG 47 implies something different. This section talks about the astral projection spell for the purposes of traveling in the Astral Plane.

Astral Projection

Since the Outer Planes are as much spiritual states of being as they are physical places, this allows a character to manifest in an Outer Plane as if he or she had physically traveled there, but as in a dream.

It further goes on to saying that high level adventurers will often prefer to travel to the Outer Planes via astral projection because dying in this form doesn’t mean “real” death.

So which is the right interpretation? Does astral projection put you back in your real body when entering an Outer Plane, thus ending the spell, or do you remain an astral projection?

Is there any mundane way to travel from the Border Ethereal of a plane to the plane itself?

Say a commoner with no spells, class features, or magic items is Plane Shifted from the Material Plane to the Border Ethereal.

The DMG makes mention of "curtains" that allow travel from the Deep Etheral to the Border Ethereal, but nothing of any portals in the Ethereal plane itself.

Is there any way for this individual to return to the Material Plane?

Can I shadowstep to the Ethereal Plane if I can see into it?

I am currently playing a Way of Shadows Monk in D&D 5e and now have the Robe of Eyes. I am loving the MadMage campaign.

Since the Robe of Eyes says

You can see Invisible creatures and Objects, as well as see into the Ethereal Plane, out to a range of 120 feet.

And Shadowstep says

When you are in dim light or darkness, as a bonus action you can teleport up to 60 feet to an unoccupied space you can see that is also in dim light or darkness.

Does this mean that I can use Shadowstep to enter the Ethereal plane, scout around and then Shadowstep back in on the following round?

Is the negative energy plane underneath Golarion?

I had always been under the impression that Golarion and the negative energy plane were on entirely different metaphysical planes. This would mean that you couldn’t simply move from one to the other using mundane means.

The Tar-Baphon entry on Lost Omens: Legends says (pg.104):

Tar-Baphon dug a portal to the Negative Energy Plane on the Isle of Terror …

To me, this sounds like he physically dug a hole to the negative energy plane. At face value, this would only be possible if the negative energy plane were somehow underneath the earth of Golarion.

I have access to many of the PF2 books, but am weak in lore published in PF1 resources. Is it true that the negative energy plane is literally below Golarion?