Question: How do you forward streams from one connection to the other?

I'm not sure if this is the best place to put this, but I can't find any other kind of forum.

Say I have two separate RTC connections with two separate peers. How could I take an incoming video (or audio) track from one peer and send it as an outgoing track to the other peer? Kind of like a lightweight SFU.

This might be obvious, but I'm kind of new to WebRTC.