WiFi signal is received by both. Packages are ignored if they are not requested by either one of the systems. You can also receive anyone else's packages while you are using your internet but ignoring the ones you don't need. (If interested try aircrack-ng.)

Only if it’s unencrypted, of course. Rarely the case nowadays.

If it's important but you got time, you could always save the packets and crack them when quantum computing comes out for consumers. You have to wait a couple of decades probably, but maybe it's worth it

NSA is already taking care of that in Utah.

Your computer will still receive the packets from the radio layer, it just won't have the right key to decode the other recipient's traffic.

Of course if you don’t sit in a Faraday cage you can receive any electromagnetic waves around you. But you can’t actually receive other users IP packages.

Yeah, the client OS will reject the "bad" packets destined for the other device as unknown.