This is a common misconception.. you and your neighbour can configure the same channel, you cannot successfully transmit at the same time on the same channel within range. Nor can you and your own AP successfully transmit at the same time on the same channel.

When you and your neighbour _appear_ to be transmitting at the same time, each adapter is actually spending most of it's time waiting for a clear medium and for various backoff timers to expire before attempting to transmit.

"Appear as noise" is not defined for Wi-Fi adapters. There is only "I received a frame addressed to me and acknowledged it" or "I sent a frame and either did or didn't get an acknowledgement back from the receiver". Receivers do not know why they didn't receive a frame, or, if they received a corrupted frame, why it was corrupted. They just wait for a retransmit. Senders ordinarily wait a certain time to receive an acknowledgement, and if they don't, the start the transmit wait cycle again. But they often then reduce the data rate to increase the odds of a successful transmission.

I'm glossing over some complexity here, because there's a sender and receiver to consider, and each has a different view of the RF environment, but the point is always correct when all transmitters and receivers (lets say the 2 APs and each has 1 client) are in audible range of each other. And this is most of the time. Note that "audible range" (where the signal is such that the medium is deemed as busy by the adapter) is much larger than the "usable range" (where data can be transmitted at reasonable speeds). So transmitters create interference in a much larger area than they actually operate in.

That means your neighbour transmitting at 6Mbps to his AP will indeed degrade the performance of your client who wants to transmit at 600Mbps because your client has to wait ~100 times longer for a clear medium.

It is not even switched on in some early version of WiFi 7 router and receivers.

As a general rule of thumb, the best version of WiFi x will only come with WiFi x+1. So for all the problems to be solved and ironed out on OFDMA it will be WiFi 8 then. And for all the promises of Ultra-High Reliability, it will have to be WIFI 9.

WiFi is clearly moving more towards like 4G and 5G with every version. I just hope someday that it really is good enough where there are many people using it at the same time.

Yes, and before that MU-MIMO is also an improvement to the problem. Still only 1 transmitter at a time, but multiple receivers.

Yes, that helps quiet a lot in practice because in most places there's limited "frequency-domain" capacity (i.e. free channels) but plenty of "time-domain" capacity, (i.e. free air-time). So even if you are sharing a channel with 4 other APs and their users, everybody may subjectively feel the network is fast. When chopping up the time domain into nanoseconds there's just a lot of idle time available, even if clients are pulling down files at 600Mbps.

But at a fundamental level, the channel space (~60 across all bands best case) is extremely limited but the potential growth in transmitters is unbounded. It's like a linear hack to an exponential problem. It seems to work at first, but under very high load conditions performance still degrades ever faster until it falls off a cliff. Then there's all sorts of complex dynamic behaviour like the hidden node problem to add to this, but it all boils down to needing air-time and SNR.

Yeah 6Ghz freq doesn't have DFS channels which remove a lot of usable channels for 5Ghz. Unfortunately it'll be a while until most devices support 6Ghz.