Linux 101 stuff. Questions are encouraged, noobs are welcome!

Thank you for your time!

1. Are desktop environments changeable once the OS is installed? Can I switch between Cinnamon and KDE Plasma, for example? Or should I decide on one before I settle for a distro?
2. Coming from Windows, I'm used to my hard drives just being "there", no matter what program I'm using. I need to explicitly set permission levels in order to restrict access, but by default I can access anything anytime. The last time I tried Linux, I couldn't get the program I was using to "see" my hard drives in order to open a file, despite the drives being accessible in the OS file manager. How does this stuff differ between Windows and Linux? What basics should I know about hard drives, e.g. what the hell is "mounting"?
3. What are the crucial differences between the different package managers (e.g. Flathub, Snap, etc.) and why should I choose one over another? And like with the desktop environment question, can I switch app stores at will, or do I need to commit to one before I begin? I've read a lot of very angry and passionate comments in passing related to the various package managers, but none of it was accessible/comprehensible to a n00b like me.
4. Can you recommend a resource - be it a CoolGuide image, video series, a PDF, a website, whatever - that will explain in simple terms the Linux way of doing Windows stuff? (Like with my hard drives question, for example.) "If on Windows you do X, on Linux you'll have to do Y" kinda thing, but for everything, no matter how basic.

Thanks for reading 👍