How do I clean up unwanted files and junk on my MacBook Pro?

Consider if your situation uses iCloud+ or online file sharing, then

change the parameters of that, before deleting any shared files.

some ideas (w/ unresearched) abound

https://duckduckgo.com/?t=ffab&q=

how+to+save+to+backup+macOS+files+offline&ia=web

Online storage, shared; needs to be isolated Offline, to save any.

Backups to external USB and/or local hard drives is always good.

About This Mac -> Storage -> Manage

If you accept the recommendations offered you will save space