You can read about the filesystem here https://linuxlap.com/linux-tips/linux-file-system-structure/. At home, I rarely go outside my home directory. Outside the usual folders in /home/user (~) like Documents, Downloads, etc., I mostly find myself in ~/.config and ~/.local/share looking for files that desktop programs store. Or for whacky programs like the email client Evolution, you can find the entirety of your IMAP emails in ~/.cache and have to redownload all your emails with a new PC because who backs up their cache folder? (Or angrily switch back to Thunderbird and never use Evolution again.)

At work with proprietary software to support, it’s at /opt.

You can check where programs are installed with which, ex. “which firefox”. Flatpaks are stored in different directories and ‘which’ won’t find them. Better to manage those with warehouse and flatseal than mess with the files directly.