Computer Help and Support

Phones will use storage for who knows what.

You could try asking the same question in duckduckgo search. You obviously aren't the first to run into any given problem and neither am I.

There may also be an app that indicates and possibly cleans out duplicates and mystery operating system use.

My own approach, since I haven't had problems lately, is to regularly transfer all photos to the computer, and I can clean out the phone.

If you have a "memory usage" function, take a look. Messages often contain videos that people send, and messages and emails can take up gigabytes of space. You can delete unneeded emails and videos in messages. Deleting an email will delete any video contained in it.

Do make sure you back up before deleting things. "just in case"

I understand that newer android phones don't have sd cards. From what I read, the phone makers would much prefer that you buy internal $$$$ memory from them or u$e cloud $torage. The greedy bastards copied Apple.

Though, in recent Apple models, you can actually plug in a flash device via adapter that lets you copy photos (and other stuff? I haven't tried this in a long time) via the "files" app.

You can do it, too.
https://www.howtogeek.com/129800/how-to-use-usb-drives-with-android-phone-or-tablet/
https://easytechsolver.com/can-i-use-a-usb-c-flash-drive-on-my-phone/
https://www.androidpolice.com/use-phones-usb-c-port-for-extra-storage/

within software and hardware limits. Do check before trying.

HTH