So I'm a little late to the party here [as usual], but for posterity I'm going to jot a few things down for anyone else that may come along confused.
1: Programs you install are not the only ones that use disk space. A good example of this is Windows System Restore. It makes incremental backups, and that takes up space. Another good example is the Windows Page file. The Windows page file is where programs used to cache data when your RAM was full [we're talking a long time ago, of course] These days it's still very well integrated, and even though You'd THINK 8-16 gig of ram would be enough. . people still set their programs to use and require the page file. You also have system processes like Windows Superfetch, which monitors and "shifts" data around to make sure frequently used data is most accessible [at least on mechanical drives, but it still seems to organize stuff initially on computer setup] as well as search indexing. A lot of times this can involve copy/pasting of data [again, on a mechanical drive, but this is a HUGE part of why a new computer setup seems to run so hot and slow for the first 24 hours or so, I'd swear it]
2: MicroSD cards.
A: You may have to go into disk management and MOUNT the drive. You may also want to make sure it's formatted as NTFS or EXFAT. Fat32 is fine, except that you will not be able to write files bigger than [I wanna say the limit is 2 Gig, it's been awhile] but if "My PC" doesn't show your drive, it's not properly mounted by the O/S.
B: An SD card is *NOT* an additional hard drive. There are things you can do with it, like store movies, move your onedrive folder over to it. . but you are NOT going to be using it for gaming. Windows also does not allow removable drives to be used for some tasks [app installation, etc -- this ain't Android] The read speeds on SD cards, ESPECIALLY MicroSD are WAAAAAYYY too slow. If you want to go that route, the best advice I can give for gaming storage is as follows:
cheaper: a USB 3.0 external drive. Not the fastest access times, but it'll get you by
Better: Get a Samsung T-3 or T5 USB-C external SSD. I use a T-3 500GB for my steam library, and it works flawlessly. Sure, it'll set you back a few bucks, but that's a small price to pay for massively expanding your storage space without sacrificing performance
Hybrid: Get an internal 2.5" ssd and an external USB 3.0/C enclosure. Price may vary. Performance should be fine. Dunno, never tried, but it's an option.