Formal sessions of the full houses are rare. Most legislation is routine, and each party sends a few people to make sure the other doesn’t sneak anything through, and they pass everything on a voice vote. Unless someone asks for a quorum call, they can ignore the fact the room is largely empty. Full sessions are only for really important business.
The real work is done in committees and conferences, many of which are scheduled in advance and open to the public, and occasionally televised.