https://syncthing.net/ <- like this :)
Free, opensource, works on computers and phones, can in most cases puncture nat, supports local discovery (lan, multicast).
No googles, no dropboxes, no clouds, no AI training, no "my kid likes the wrong video on youtube, now our whole family lost access to every google account we had, so we lost everything, including family photos", just sync!
(not affiliated, just really love the software)
The problem with syncthing is that you need to download the whole folder to your device. Google drive is "streaming"
This is my go to solution for code sync across macOS laptop, Windows VMs, and Linux VMs to build and run/debug across environments. Unless something has changed, exclusions of build artifacts was always an issue with cloud sync providers. I have been doing more cross compilation on macOS, copy and run on those other machines lately for prototypes, but for IDE based debugging it’s great to edit local or remote and get it all synced to the machine to run it in seconds.
Sadly it doesn't have a great official solution for mobile devices.
The only issue I have, with this amazing piece of software that I heavily use across multiple devices, is management of sync failures and exclusions via the UI. I have been using it for long enough to know the tips and tricks but it would be great for the web UI to allow easy management of conflict issues and the ability to mark files/folders as exclusions in a friendly manner.