How do I delete save files from my file explorer?

@Decomposingdolls  Open Documents > Electronic Arts > The Sims 4 > saves, and delete what you want.  I'd suggest deleting the backup versions of your saves first, as in, the files that end in .verx rather than the ones that end in .save.  That way, you'll have one copy of each save you've played while still probably reducing the size of the saves folder by over 80%.  Leave the most recent backups though, the ones for your current save, in case you need them.  You can sort by date modified to distinguish these from older saves.

To delete, just drag the files to the recycle bin, then open the bin and select Empty when you're ready.

The Downloads folder on your computer is another good place to look for extraneous files, for example old installers or .zips for which you've already extracted the files.

Finally, external storage options are cheap these days, so it's worth considering getting a 1 TB HDD or a 128 GB USB or whatever suits your needs.