An email alias is a separate address that delivers to an inbox you already read. The sender sees the alias; you get the mail where you normally get mail. Nothing about that is complicated — the useful part is what becomes possible once each company has a different one.
Give one address to one service and three things follow. Spam becomes attributable, because only one company was ever told that address. A leak stops cascading, because the breached list holds an address nothing else uses. And you get a lever you can pull alone: switch that address off and the sender loses its route, without needing their cooperation or a regulator's.
These guides cover how aliases work, how they differ from disposable addresses, what disabling one actually does at the protocol level, and the question most people only think about after they've handed out three hundred of them — whether the addresses survive the service they were created on.
They are not anonymity — you typed your real name into the form next to the address. They do not delete anything from a company's database. And they do nothing about the exposure you already have; they change what happens from now on.