Q: what's wrong with my mac

And your question is?

I suspect that you are trying to tell us that you do not have that many applications installed?  That display is not to be trusted.  Often, but not always, a Spotlight reindex will correct it:

https://support.apple.com/en-us/HT201716

If you want to know exactly what is in your MBP,  download from the Internet OmniDiskSweeper and Grand Perspective (both free) and open them. They will show all of your files and the respective sizes. Transfer to an external HDD the files you do not want on your Mac or delete them.

Do not forget to empty trash. Only then is space allocated for additional data.

https://www.omnigroup.com/more

http://grandperspectiv.sourceforge.net/

Ciao.

yep, iMovie takes a lot of memory because the movies you make take a lot of memory. I used to edit videos all the time and i could never do anything else when i had a video open. The best solution in editing videos is to get as much RAM as your machine will take. If, when it hangs, it hangs only in iMovie, then it is probably just not having enough RAM to work with. In the meantime, shut down all other applications each time before you open iMovie. The longer the movie (or the more footage), the more likely it will freeze even if everything else is shut down.