Large conference, hybrid set up. Audio problems.

OK, the problem is Zoom will try to manage echo cancellation and outside noise, but with the presenter using the webcam mike, they are not close to it and I expect that zoom is perceiving the speaker as background noise.
You really need to use the same sound source as the sound system if in use.
If there is no sound system, then I would try turning off the background suppression. In the Zoom App under settings, there is an audio section and the Audio profile allows you to change noise suppression.

The weak point here is almost certainly the webcam mic, not Zoom itself. A webcam mic across a large open conference room is usually too far from most talkers, so Zoom hears a mix of distant speech, room reflections, and the room speakers. That turns into the garbled / inaudible audio remote people complain about.
 

What you are looking for is a real conference-room audio input for Laptop B, not another webcam. The common categories are:
 

1. A USB conference microphone / table mic for smaller rooms.
2. Boundary or gooseneck mics if people speak from known table positions.
3. A wireless handheld or lav mic if one presenter is leading most of the meeting.
4. A ceiling or room mic array if this is a permanent large-room setup.
5. A USB audio interface or small mixer if the room already has a PA/sound system and you need to feed that same audio source into Zoom.
 

The clean setup is: one laptop owns Zoom audio, the room speakers are connected to that same system, and every other laptop joins muted with computer audio disconnected. Multiple laptops with live mics/speakers in the same room will create echo and cancellation problems.
 

Before buying anything, do a quick coverage test: start a Zoom test/recording, have someone speak from the front, middle, back, and sides of the room, then listen to the recording. If some positions disappear, you need better mic coverage. If everything sounds roomy or washed out, clap once in the empty room; if the clap rings or flutters, the room acoustics are also part of the problem.
 

Full disclosure: I built RoomScore because I kept running into exactly this kind of hybrid-meeting room problem and needed a repeatable way to separate mic coverage issues from room acoustics issues: https://roomscore.tech/conference-room-audio-testing-tool/
 

I would not start by buying the app or swapping hardware blindly, though. The first fix here is simpler: stop relying on the webcam mic and give Zoom a dedicated room audio source.