Open the logbook and use C-h m to find out what the major mode is. You’ll also find there any documentation for the mode, the keybindings that are active, and a link to the source code should you need it.
As a general rule, Emacs is intended to be self–documenting; you can always ask Emacs what is going on, and it always gives you as much information as it ...
It depends on what kind of changes you make. There's nothing special about code invoked from a hook.
If you set a global variable it remains set until something sets it differently or makunbounds it.
Your question is general, so the answer is general. If you pose a specific question, e.g. with some code, specific answers can likely be given.