How is IEEE 802.11(Wifi) made robust to interference?
It depends what you mean by interference.

If you mean data passing through router A 'appears' on router B's clients, then no, this doesn't happen. Each router &lt;-> client link is encoded to label it so that it only gets delivered to the right destinations, even for unencrypted links.

If you mean that RF from router A degrades the signal to noise ratio (SNR) at router B, trashing the throughput, then yes, this is the big problem with closely physically located WiFi links. There are only 11 RF channels defined (around 2.4GHz), and they overlap, so there are only really 3 clean channels 1, 6, and 11. If you have multiple routers operating in the same channel, then numerous data packets will be corrupted and have to be resent.

Microwave ovens, bluetooth devices and cordless telephones also share the same spectrum, and create interference.

A case in point. I have an 'n' router and client, that should be capable of 150Mb/s. When operating on the same channel as three of my neighbours, it drops to 1Mb/s. Move channels and the data rate restriction becomes my LAN or WAN connections.

Use linSSID if you're on linux to have a look at your local WiFi environment.