Pfsense with Unifi USG

Ok I’m a little bit of a networking NOOB.

Looking at the setup in the attached picture, I want to add a pfSense router and keep the Unifi USG in my current network, as I like the dashboard and device mapping Unifi provides. Is there a way to make the Unifi USG passthrough the internet from the pfSense router, so effectively just use the USG as a switch or could I use the Unifi USG to create a VLAN after the pfSense router?

If so can someone explain this set up step by step for me, in simple terms, so I can go research the right topics?