Many things can bind to a single type of receptor. When I studied psychopharmacology in college there were a lot of great analogies, one of which comparing a receptor to a lock, and a molecule that binds a key. However, a traditional lock and key is too simple, imagine a lock that can sense every part of the key (a full 360 / 3-D analysis). Since there is so much variability in different molecules, with polarity/charge, etc. A lock and key is far too simple of an analogy, but if you can imagine a mind blowing, highly complex version, you can get closer. And rather than the lock opening a door or something, imagine it turning on a machine. But based on how the lock is turned (or activated) it can change the function of the machine, or turn the machine off/on/partially on/off, etc. Completely changing the function of the machine based on what is needed.

A receptor can be activated in so many varying degrees, and each variation causes it to do something different within the cell. There are so many different cascading mechanisms within the cell based on how the receptor is activated. Serotonin does something very different than psilocybin to that receptor, and thus activates a different cascade of activation within the cell.

Here is a basic image of a signaling cascade within a cell from a single receptor. Notice how there are multiple pathway possibilities from a single receptor within the cell.