Will all the devices (which are labelled with 802.11x) use CSMA/CA?
On a shared medium, you need something like CSMA/CD or CSMA/CA. Because Wi-Fi devices cannot both send and receive at the same time, they cannot use CSMA/CD to detect the collisions, so the use CSMA/CA to try to avoid collisions.
How do each of these WAPs know which sub-carrier frequencies ...
Wifi-6 will change that for us. This will allow all devices to get a chunk of the bandwidth depending on their needs, and there won't be a need for taking turns to talk to each device on the Wifi network, making it super efficient and super fast even when there are a lot of devices.
A wireless radio network, is a shared medium. That means that only one sender at any time can be permitted to transmit. Multiple, simultaneous transmission would interfere with each other and garble the data.
However, packet-switched networks transmit data in frames, (practically) up to 1500 bytes in size. Transmitting 1500 bytes with e.g. 600 Mbit/s takes ...
It looks like both monitor and promiscuous modes of tcpdump should get you a lot of traffic: