~~I'm using Syncthing on Fedora and Syncthing-Fork on Android. ~~
I'm fairly sure I followed the instructions on this page https://linuxiac.com/how-to-set-up-and-use-syncthing-to-sync-files/ But I will admit some of it doesn't look familiar.
There is also GSConnect. I haven't used it so can't comment further. https://www.howtogeek.com/keep-linux-pc-and-android-phone-synced-with-gsconnect/
Edit: with Syncthing-Fork's new mystery dev, I've decided to discontinue using that service. I decided to use Localsend as recommended by SideofBurritos. The app doesn't auto sync like Syncthing but is very easy to use and seems fast.