Android phones have the Android OS from Google(or a modified version by the manufacturer of the phone).
We can install other unofficial versions of Android(modified by other people).
But it is risky, as there is a chance that it can go wrong 'brick' your phone. It's named so, because on failure, your phone may become unresponsive and would only be as useful as a brick or paper-weight.
There's hard-bricking and soft-bricking, where the latter is used when the bricking can be fixed/reversed.