The app syncthing-android does not collect any user information. It does run Syncthing, which does expose some networking information to work and may collect usage-data only if you agree to it or run betas/release candidates. This documentation article has specific information on this: https://docs.syncthing.net/users/security.html
The app requires the permission to access the location in the background, but it never does look up your location. The location permission is required to look up wifi SSIDs, which is used to enable/disable syncing on user configurable wifis.
