Read other materials first to familiarize yourself with wireguard and wireguard on FreeBSD / Android: genneko has a nice writeup that is easy to follow and much better formatting here:
https://genneko.github.io/playing-with-bsd/networking/freebsd-wireguard-android/
Backup your entire pfsense system, or run this on a vm after a good snapshot. Failure to do this can easily break you, you have been warned. This is entirely unsupported. Wireguard on PFSense is experimental at this time. Nothing has been tested by me on non AMD64 arch, so unsure if arm pkgs are available. Backup your package database just in case since this impacts core PFSense runtime dependancies.