This is the section of the layout diagram with the four white boxes, in the bottom right. The white boxes represent common receiver types. The top white box labelled CRSF is for Crossfire receivers.
The section of the flight controller marked by a yellow line indicates the pads used by receivers.
The reason for this is that some receivers don't include telemetry, so don't use the TX2 pad, and SBUS receivers send an inverted signal that needs to be handled slightly differently.
No, you can't connect multiple receivers. Instead many receivers have two antennas and automatically use the one with the best signal.