How does Wi-Fi deal with multiple frames received at once?
Talking about IEEE 802.11 a/g/n/ac… here.


  As far as I know, WiFi devices are able to receive multiple frames at once with no problems.


That's wrong.

WiFi isn't designed to allow working with collisions. It has systems in place to avoid these.

So, a single WiFi device can only receive a single frame at once. It might be that complex devices were designed to work on multiple separate WiFi channels at once, but that's definitely not the norm, and it's actually like multiple devices doing multiple independent things at once; the frames don't collide.


  How is this possible?


Aside from the system being neither specified nor designed to allow that:

Since a collision of two frames means that the preamble of the second one would be heavily damaged, I doubt it's overly possible practically.

Principally, even systems that weren't designed to allow collisions (those would typically be CDMA systems) can achieve reception of overlaying frames if the signal strength of one is significantly better than the other:

In that case, the receiver might be able to actually correctly receive one, recreate the "unininterrupted" signal as it would have sounded to itself, subtract that from the receive signal (that it has magically saved somewhere), and then receive the next weaker one. We call that technique Successive Interference Cancellation; again, as far as I know, that's not a part of any WiFi standard.

Also, MIMO/beamforming techniques can be used to "detangle" signals arriving from different partners/paths at the receiver, but that is a whole different kind of business.