Watching people talk about things you literally have zero knowledge about.

Perhaps…
There are several ways to bodysnatch. But doing so permanently would involve making significant changes to yourself, or losing something significant.
If you were feeling like becoming a creature, then sure, ghouls are the best examples of a similar existence, being able to replace and imitate the beings they devour, with all their memories and most of their abilities. There are worse things, that are capable of even more than this, predators from beyond. Some have sought such power.
Another way to do it would be a permanent form of possession that would transfer your soul to the new body. Anything that’s purely physical will be lost, and replaced by your new vessel. There are some things that might carry over, like curses.
Both and more. There are internal and external parts and that also really depends on what kind of caster you were. Magic is the blood of chaos
As a sorcerer, magic is something you’ve awakened in yourself, an ability that became innate like a muscle or organ, but it depends on both the soul and the body. Your body is adapted to magic, in a way most other casters won’t be, you will be a vessel for magic and it will flow through you like blood. Something similar will happen to your soul, and those two things intersect, they are part of each other, sever one and you will lose a lot. It will be possible recover that power, but it is like, an actual wound, like tearing off your own damn head to tape it up on another body, it will be slow to heal, and your new body will have to adapt, new channels will need to be made.
If you were a divine caster like a cleric, well Faith is entirely spiritual, it tends to have some effects on your body as well but… say you did exchange your soul with someone, you will find it much easier to adapt and use your powers to the fullest extent again, because your soul is still the same and the physical aspect of your magic is relatively minor.
That can happen. Whether as echoes of them, a presence left behind as they passed on. Or imitated by some manner of creature.
It’s an interesting question, I have to admit I didn’t really think about it before.
If it was a wizard for example, its memory would retain patterns and rituals, the brain would retain the conditioning they go through to be able to cast in the first place, things like that. If it was perfect, it might work seamlessly, but having even a 99% right copy would be incredibly unlikely. The nervous system is an important part of the casting process for wizards, so having the brain, spine and nerves would make for a better copy.