Linux

I use kdeconnect with this script: https://github.com/Samueru-sama/kdeconnect-any-filemanager

Simply install flatpak then install warpinator

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

For secure private transfer use the Warp flatpak in Linux and Worrmhole William in Android.

There are some browser based solutions like sharedrop.io and file.pizza. I haven't had the latter work for me though, not sure if it's still functional. They work through WebRTC to discover local candidates for receiving files, the same way that video calling typically finds the best connection.

Security

ShareDrop uses a secure and encrypted peer-to-peer connection to transfer information about the file (its name and size) and file data itself. This means that this data is never transfered through any intermediate server but directly between the sender and recipient devices. To achieve this, ShareDrop uses a technology called WebRTC (Web Real-Time Communication), which is provided natively by browsers. You can read more about WebRTC security here.

Mostly because I'm not the most competent techie, I've been using VLC between my PC and iPhone, for moving "books" around on devices that are very out of date.

snapdrop.net if on the same network

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

wifi file explorer pro apk

Maybe not the fastest or most secure but you could set up a torrent. Some clients are open source.

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.

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.