Available since August 27, 2020 on Pokémon GO, the Mega Evolution feature was already known from other games in the license since the Kalos generation. The principle changes a little since now you will have to collect Mega Energy in order to Mega-Evolve your creatures.
To obtain them, you must participate in Mega Raids. Below is a list of the Raids currently available, and the Mega Evolutions that will arrive in the game sooner or later.
List of available Mega Evolutions
List of existing Mega Evolutions
These Mega Evolutions are not yet available in the game but will be sooner or later.