Well, since microsoft is absolutely horrible, here's the solution to my own question that I stumbled upon - in case anyone else wants file explorer to NOT show thumbnails for word docs, but show thumbnails. (I can't find a solution for pdf files, but they're not the major problem for me. And I assume this will work with other Office files.)
I want to see jpg thumbnails in file explorer but when I turn on the thumbnail option (in options of file explorer) then all the numerous current word docs i'm working on that are saved on my desktop --- half of them turn into thumbnails and half are still icons. Not logical at all.
If you want to fix this...the solution is hidden in Word - not file explorer (*sigh*).
Follow the instructions on this place to find the two hidden places to "create" thumbnails in Word. https://office-watch.com/2022/thumbnails-office-word-document-tricks/
The first (in the info section that controls all files) - it was already unchecked. I checked and unchecked and saved. No change, but still worth trying.
Then for the files that showed up at thumbnails.... I had to open the file. Make a change (I just added a space somewhere). Then hit F12 to bring up the save dialogue. The little "thumbnail" checkbox was NOT checked - but I checked and unchecked it and then saved the file. (You must make a change of some sort to the file for it to re-save it and the change to take effect.
Do this to all WORD files that show up at thumbnails. Then go into file explorer settings. Change it to show icons. Save changes. Then go back in and tell it to show thumbnails.
Somehow the "brilliance" (/s) of microsoft has Word sometimes save thumbnails for files that I NEVER TOLD IT TO DO SO. But if I go back in, make a change to the file and then check and uncheck that box, the thumbnail that should have never been there will go away.
Why do we accept the so many numerous problems with microsoft? I'm changing to LibreOffice. This is ridiculous. And I've very close to moving to Linux.