It’s a single frequency for all people, including those that are on the same or adjacent/overlapping channels. If you ever used walkie talkies, they don’t work so well when more than one person is talking.

Same idea.

Wi-Fi 7 is introducing multi link, which could improve the duplex operation, but it’s far from perfect. Even if you use one channel for download and one for upload, you’re still competing with everyone else on the channel - whether they’re connected to your network or not. They can still interfere with you if you’re on the same channel but different networks.