Thanks. Much clearer now.

If you bought a private circuit from BT they'd install an NTE to terminate the fibre that unit you see if that equivalent, so the Ethernet ports on that device is their handover point to you, you can use whatever router/firewall/etc etc you want and connect it up

The FTTP modem handles the Broadband on LAN port 1 to a cable router - the HH3 for instance and the Fibre Voice Access service runs through TEL 1. You cannot swap out the ONT with another they just don't work if you do, very much a propriety piece of kit.