~~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.

The fork is going strong. Other than the occasional sync conflict to manually resolve I find sychthing works well.

You almost certainly want Syncthing

If you don't want a "same all the time" sync, and only want one-time transfer, use KDE Connect.

Syncthing!

Syncthing is great and it has served me really well. I'm going to dip in and recommend a NAS which is so great for file storage and sharing between devices in this way. My Synology NAS started me on this journey and now I'm getting deeper and deeper into self - hosting and expanding my home server.......so be warned that this is a hell of a rabbit hole to jump down.

I use SyncMe on android to sync stuff from smb shares on my Linux server. Pretty easy and simple.