How does a Battle Master’s Parry work with multiple damage types? [duplicate]

Suppose an attack does 5 piercing and 5 poison damage (or 5 slashing and 5 fire, etc.) and the player parries the attack, and rolls to reduce 6 points of damage. Which type of damage is reduced first? Would the answer be different if it were 9 weapon damage and 1 elemental, or vice versa?

Parry:

When another creature damages you with a melee attack, you can use your reaction and expend one superiority die to reduce the damage by the number you roll on your superiority die + your Dexterity modifier.

This matters if, for example, the character has resistance to one of the damage types.

The kindest interpretation (and one that does make some “sense”) would be that it reduces weapon damage first, and if it reduces the weapon damage to zero, none of the additional damage is done either, but that seems over powered. Unless someone can unambiguously point me to rules that support this interpretation, I prefer an interpretation where the total damage reduced is as rolled.

Ranger Beast Master’s attack bonus

The Ranger companion says to add your proficiency bonus to the animals attack roll. Does this replace the animals proficiency bonus or add to it?

Ie does a 5th level ranger flying snake get +9 bonus or +7? The snakes natural +6 is it’s +4 dex bonus and it’s +2 proficiency bonus so does the ranger replace the +2 with her +3 bonus or add her +3 to the +6?

The players hand book says not to add your proficiency more than once which might suggest the +7 but technically the snakes proficiency and your proficiency are not the same bonus which would suggest the +9.

Does the Beast Master’s Primal Companion feature allows the beast to take two Actions in a single turn?

I watched a video from Dungeon Dudes on YouTube where they rated the Ranger subclasses. Around the 23:18 mark, they mentioned that the new Beast Master’s Primal Companion feature from Tasha’s Cauldron of Everything can allow the Ranger to forgo one of their attacks to allow the Beast to take the Attack action, on top of the action that they can make using the Ranger’s bonus action. The Primal Companion feature description says the following:

Primal Companion

[…]
In combat the beast acts during your turn. It can move and use its reaction on its own, but the only action it takes is the Dodge action, unless you take a bonus action on your turn to command it to take another action. That action can be one in its stat block or some other action. You can also sacrifice one of your attacks when you take the Attack action to command the beast to take the Attack action. If you are incapacitated, the beast can take any action of its choice, not just Dodge.
[…]

As far as I know, a creature can only take one action on their turn. Is this a specific ruling for the Primal Companion since it doesn’t have its own turn because "it acts on your [the Ranger’s] turn?" Also does this mean that the Ranger can break up their movement, actions, and bonus actions with the Beast’s on their turn, essentially controlling two characters freely? In the video, Monty mentioned that a level 11 Beast Master Ranger can make one attack and have the beast attack four times which is crazy in my opinion.

What does the Exotic Weapon Master’s Uncanny Blow require?

The Exotic Weapon Master (Complete Warrior, p. 30) has, as one of its optional class abilities, the Weapon Stunt Uncanny Blow.

Uncanny Blow gives a benefit to the Exotic Weapon Master "[w]hen wielding a one-handed exotic melee weapon in two hands[.]"

To trigger this benefit, must a creature wield a one-handed exotic weapon that does not offer a two-handed martial option?

Would an Exotic Weapon Master without Exotic Weapon Proficiency: bastard sword still be able to benefit from this weapon stunt? The weapon stunts themselves only require Weapon Focus – and since the Bastard Sword is also a two-handed martial weapon, a character proficient with martial weapons could presumably take Weapon Focus in the bastard sword, always intending to use it in two hands. That character still has Exotic Weapon Proficiency with the Spiked Chain, and so the character does fulfill all the requirements for the class and the stunts.

But, if the character always plans on using it as a martial weapon in two hands, then it is never being used as an exotic weapon (ignoring that putting two hands on such a one-handed exotic might make it non-exotic anyway) . . . So can the Exotic Weapon Master ever benefit from Uncanny Blow with a bastard sword?

Should I get the Dungeon Master’s Guide, the Player’s Handbook, or the Monster Manual? [duplicate]

So I’m a fairly new player in the game Dungeons & Dragons (D&D). I’m known about the game/played a very loose version of the game a little over a year ago and I started getting serious about ten-eleven months ago. Currently, I am still learning about the game, its concepts, and how to play. Something I would love to try is DMing. But there are a few problems with that:

  1. I’m still not a master at the game and I’m still learning
  2. I’m not sure if I have all the resources.

So I’m asking you guys! What would be the most important to get for an inspiring world builder and D&D player? The Dungeon Master’s Guide, the Player Handbook, or the Monster Manual? I currently have some beginner level stuff that just teaches you the basics, but what if I want to go deeper?

Can I apply great weapon master’s -5 atk, +10dmg feature to a bonus attack?

Say I trigger the bonus attack granted by GWM’s first feature, (bonus attack upon crit or killing blow). Can I then apply GWM’s second feature (-5 atk, +10 dmg) to that bonus attack?

As a barbarian at level 5 (extra attack) , this would mean that I could use this feature a total of 3 times — that’s some heavy damage.

How to start preparing for Masters thesis?

I am a first year Masters student in Computer Science from India. I am due to begin work on my Masters thesis after 2-3 months. Currently, I am in a big dilemma in choosing the area of my interest.

Last semester, I took courses in Machine Learning and Digital Image Processing. I developed an interest in the area of Computer Vision (more so on my whim), and I have thought of a few problems which might have interesting applications, but I am unsure of its feasibility.

Please provide some links to journals/articles/blogs/MOOCs which could help me explore the domain in a targeted way, without getting lost in the sea of content available on the internet. On a Google search, I am finding it difficult to sort and select the right material.

Thanks!