Fortnite and Bots?

Is every lobby just bots? Or even players? And are the bots going too?

The lobby is filled up with bots, but for the most part it's normal players. Whether the bots go away I do not know.

Which platform?

Ps4

That depends on your skill. If you are rather poor, your rounds will be filled up with more bots, so that you meet equal players. At the higher skill levels you will hardly fill up with bots anymore.