I always had a hard time wrapping my head around the setup. I mostly use Nextcloud for sync now.
I use it to transfer large video files and 40000+ images from my phone to PC it transfer fast then normal copy between systems
I connect my PC and phone in tethering mode and open syncthing on both devices
I get 30mb+ per second transfer speed
That's one of the advantages with p2p-ish networks, if you're on the same network, devices can often disover each other and communicate directly, bypassing the bottleneck of an internet connection like traditional cloud services have .
it should be in the repository, you can use web UI, default is
http://localhost:8384/
https://docs.syncthing.net/intro/getting-started.html
https://docs.syncthing.net/intro/gui
It's really annoying to back up anything on Android as the fucking SAF doesn't allow me to back up root folders, so I have to back up everything individually.