Where can the Changed Files For V9 be found at please?

Hi,

I am currently using 8.9.7 version for the shop theme. However, I needed to know if
I should upgrade it the latest version, and if so where are the files that have been changed, please?
I also need some help on if I upgrade to this new version, will all my current modifications to the
menus, categories, etc be altered out, and thus need to be put back into place with the upgrade?
Also, do I need to change out my PHP on my hosting to use the latest version, and the child themes too, please?

Sorry for so many questions, but I always seem to break my version when upgrading and took me ages to get it back to functioning order again, so wanted to ask upfront this time around.

Hope someone here can help me out here, much appreciate it!

libby