Lost in the Mojave OS Desert

Back up your data, check the application compatibility table, click here, and then on the download link for a newer OS. That model's maximum one is Sonoma.

Catalina and above won't run 32-bit software and split the functions of iTunes up; use Go64 as well.

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1. Don't want to appear condescending, but people do sometimes forget, so - did you empty the Trash after deleting files?
2. Are all the files ones you need to use (as opposed to say, older work files you need to keep for archival reasons)? Can you copy some to an eternal drive and then delete the copy of them on the Mac?
3. I can't recall if this affects Mojave or only newer versions - but I think it is a quirk of the APFS file system so it might - sometimes after trashing files and emptying the trash, it can take a number of hours before the computer fully recognizes the recovered free space