How to get apps to see USB flash drive?

I have plugged a USB flash drive into my phone. I get a pop-up and can see the files from a file manager of some kind that came up when I told it to look at my files from the notification. But I can’t see the files from any other apps where I really need it to show up. I tried from a third party file manager, but when I try to navigate into the drive it says I need to grant permission on the next page, but the next page isn’t the file manager! When I try any other app such as Google Photos, it doesn’t even appear to detect the drive or show it to me!

How do I access files on the USB flash drive from apps on my phone?