Many matchmaking systems feature a ranking system that attempts to match players of roughly equal ability together. Games with ranking will usually offer unranked sessions for players who do not want their performance to be recorded and analysed. These are kept separate so that ranked and unranked players do not mix.

Is Fortnite full of bots?

Without the player level in the equation, Fortnite usually adds bots to lobbies when there arent enough players in the matchmaking pool. Considering the size of Fortnites player base, this is hardly ever an issue unless you tinker with your matchmaking settings.