Hence, he is asking questions here.
It's a manual lever that de-cocks the hammer. Some guns like the Sigs, the decocker drops the hammer. Other guns like the Beretta 92FS it will drop the hammer and disengage the firing pin.
Why? As long as he was told the safety rules, why does he need to know how the gun works?
Do you know the mechanical principles of all the components of your car? Do you know the circuit design of the computer you are using?
As far as striker fired vs. hammer fired. 99% of hammer fired guns, ie DA/SA offer second strike capability. IE if you pull the trigger and the round doesn't go off for some reason, you can just pull the trigger again and it will cock the hammer and drop the hammer again. Sometimes primers just do go off on the 1st try.
With a Glock, you would have to pull the slide back a bit to get it to reset the striker/trigger so you can pull the trigger again.
Guns like the 1911 and Browning Hi-Power are hammer fired, but are Single Action only, and do not have second strike capabilities. You would have to thumb the hammer back to try again or rack the slide to eject the failed round and load the next one in the mag.