Why is docking a cruise ship in a busy port so challenging? [closed]

While watching a cruise ship on the port, I then saw some smaller ships around the big ship and guessed they were helping the cruise to dock properly but i was not sure. This got me interested in how a large cruise ships enters and docks to a port. My question is what are the challenges faced by the big cruise ship to enter a busy port and then handle its huge weight and momentum to avoid any accident? Were the smaller ships I saw really helping the big ship but how can they? I mean they are so small like mouse in front of dinosaur. What was the purpose of using small ships and why can cruise not handle on its own?

How to make a mimic a challenging fight for high level party?

After posted this question, it turned out that mimics might be weaker than I though first.
As said in this linked question, I really like mimics and I’d like to use “a lot” of these (without going too far and make my players paranoiac) even when party level is increasing.
Also in this linked questions accepted anser comments, Erik told me that :

a basic Mimic probably won’t ever challenge a high level party, but there are similar monsters of higher power levels, or you can make your own. …

So I now have another question : How can I make mimics a real challenge for a high level party ? Which monster would be similar to what a higher level mimic would be ? (a party of 4/5 players)

I’d like mimic (or similar monsters) to be a challenging fight, maybe not deadly, but at least interesting fight, not over in less than two rounds.
If necessary or easier, you can split the answer to address to floor level (i.e. : level 5 to 10, 10 to 15, 15 to 20, 20+).
As I’m very new to DnD (only started to buy basic rules, read linked question and this one for context) and even if I read a lot of this RPG stack Dnd questions, I’m not very aware of how to make changes while leave game balanced. So please be clear about how you managed to make it fit to another level (if so).
It also don’t really matter if it’s not RAW as long as it isn’t game breaking.

Answers have to be based on prior experience about using mimics at higher level or at least using monsters at higher level than their basic one.
If the answer is based on the use of these similar monsters, please back it with some in game experience about it, as a player or a GM, to support the fact that it’s similar to a mimic.

How challenging did you find Real Analysis before the penny dropped?

I’ve been following ‘Understanding Analysis by Stephen Abbot’ and have been struggling with exercise questions even after an in depth read of the chapters. I have read numerous statements by authors that ‘if you can’t do the exercises, then you haven’t grasped the material properly’, but I feel like I have grasped the concepts. I’ve read many ‘how to’ books to finally understand higher level math and persevered with a ‘never giving up’ attitude, but most exercises really throw me off. Even those in the first chapter which are meant to be on basic preliminary material.

Is this just the nature of studying Real Analysis? I’ll be starting Abstract Algebra soon and have been advised to purchase ‘J.B Fraleighs introduction’. Do other higher maths subjects cause the same anxieties as Real Analysis?