Think of Zerotier as a virtual switch. Your phone and your PFSense can be connected to this switch, via the ZeroTier software. They will also each be assigned an IP address on the virtual ethernet port that shows up on the phone and the router, which is how you can access the router from your phone.
Now, like VPNs, when you connect to Zerotier Central (assuming you are using that service) your phone can be pushed a route like “192.168.10.0/24 → 172.23.0.5” (if 172.23.0.5 is the ZT IP for your router, and 192.168.10.0/24 is you LAN).