If two devices are emitting a wifi signal in the same frequency at the same time and reach an antenna at the same time, how can the data not overlap?

In a wireless network, only one device is actually "speaking" at once. Each other device listens and waits for the air on that channel to be quiet before speaking. This technique is called carrier sense multiple access with collision avoidance (CSMA/CA)

An RTS/CTS exchange helps all the nodes stay in sync efficiently by providing a way for one node to say "hey, I'm going to talk for this long so wait this long" to every other node.

@Petr Abdulin is correct but I think all Wifi networks use CSMA/CA. Old 10BaseT non-switched wired networks relied on carrier sense multiple access with collision detection (CSMA/CD). Collisions don't happen on networks where all nodes are connected to a switch.

If it happens that two transmissions interfere each other (collision), then both transmissions will be corrupted and they will be retransmitted after a random delay, reducing risk of collision repeat.