Linux

I've been using SSHelper together with rsync for years and it works perfectly. You can log in the first time with a password, and place your public key to use key based auth going forward.

In addition to doing this over WiFi I also often use a usb to ethernet adapter (usb side plugged into phone) to get better performance if I'm doing larger transfers, for example copying off a large number of photos.

Edit: looks like there's a note on the play store page about incompatibility with newer Android versions. Disappointing. I guess I'll have to find another solution when I eventually upgrade my phone.

Server or desktop, and what types of files? I find that a self-hosted version of NextCloud does pretty well for keeping contacts, images, and videos in sync.

(You could run it on a Pi as an intermediary to both if desired)

I used to use stuff like AndFTP in the past for similar functions

Croc or syncthing depending on what kind of experience you are after. Syncthing if you want to have a shared folder like expert. And croc if you just need to send something. Croc has an app on f-droid, and syncthing is on the app store. Both are open source and pretty for excellent in their own right.

I use Airdroid! It's free and works very well

snapdrop.net if on the same network

X-plore on android can give file access via Web frontend in paid Version.

With that you can drag and drop files if that's what you're looking for.

samba. share a folder on pc, and on your phone use a file manager that can access smb folders in your local network, then just copy or move from or to that folder. bit of a hassle to set up the first time, but makes things more convenient in the long run.

wifi file explorer pro apk

My go to hack was quickly running a python http server and connect to it. I can't remember what the command was exactly. Something like python -m http.server or so, then connect to the ip from my phone, heh.