Populate is a flag set in chunks that instructs Minecraft to finish populating it. It is wholly controlled by Minecraft and may work differently depending on your version. Populate will add vegetation, resources, and water/lava ponds to whatever chunks Populate was added to. For some versions of Minecraft, mostly older versions, it will also add generated structures.

Populate does not work for versions 1.14 to 1.17. Populate does not work on superflat worlds. This includes worlds with endless borders as those are superflat worlds.

More info: Populate layer details.