Dual Band Wireless - How does it work?

Wireless devices that are capable of operating on both the 2.4 GHz and 5 GHz bands will first look to the 5 GHz band to see if it meets the criteria of signal strength, low noise, and other things that only the Apple engineers know.

If the signal meets the connection criteria, the device connects at 5 GHz, otherwise it will connect to the 2.4 GHz band.

Because 5 GHz signals operate on a much higher frequency, they are not as strong as 2.4 GHz signals. So, a wireless device that is capable will connect to 5 GHz when it is in close proximity to the wireless router.

A few rooms away, the 2.4 GHz signal will be much stronger (and faster) than the 5 GHz signal, so the wireless device will connect at 2.4 GHz.

The bottom line here is that you need not be concerned about any of this. A wireless device will connect to the signal with the best quality automatically based on its capability and location in relation to the wireless router.

Is the Airport Express only able to extend the range of an Apple Airport Extreme base station or will it work with non-apple devices?

The "extend" feature only works with other Apple devices if you are trying to "extend" using wireless only.