You could try finding changed config files by running:
sudo debsums -ac
Note that this won’t catch all. There are files that packages install and don’t touch afterwards. I my case for example it does catch that /etc/gdm3/custom.conf
was modified to enable autologin among other things.
Good person! This is how you learn Linux and gain experience. Trying to understand why something happened and trying to fix it using that understanding. Not “just reinstall” or worse “you should use X distro instead.”