Checking Flathub should give you some idea: https://flathub.org/en/apps/collection/mobile/1

There are more, not every application that works fine has metadata filled up (and not everything is on Flathub either).

I do use some webapps, but with Epiphany rather than Firefox.

I can use most native GNU/Linux apps on my Librem 5 like gnome-calculator, gnome-calender, gnome-weather etc. I can run Android apps via Waydroid. F-Droid works fine, too. Its default app store (https://software.pureos.net/categories) provides things like music players, OTP app, and games. Flatpak works, too.

See also: https://linuxphoneapps.org/

To add to others. You can tell Kimi or whatever these days to write you a GTK4 mobile apps for a lot of pet use cases and it will. The result will also be a few C files and a meson build file, that will build on the phone itself with just gcc/binutils (I run arch linux, so no separate dev packages for deps like gtk, etc. that I need to install). You can install opencode or whatnot on the phone itself, and tell the phone to build you an app, and it will. If you need it changed, you can just ask for the changes on the phone itself and agent will update the app's code and re-build it on the phone itself. It will not need the 30+ GiB monstrosity of whatever Adroid needs to just build a hello world app and a separate computer.

The app will also build and run on your desktop without any/many changes, if you need that.

I got a fairly nice linphone GTK4 phone frontend app this way. So it's not just for toy apps. FOSS/Linux phones are well positioned for this self-building/self-updating prompt based software development, because you don't need separate computer and shitton of SDKs to build the apps, and while phone UI is shit for manual programming, it's not at all shit for writing prompts.