Ranked system

Can someone explain the ranking system? Played my first ranked today, and was put with people who seemed like a high rank, and probably were a decent rank, for my first game. We won, and I started at Bronze which is the bottom right? So this game doesn't have placement matches you just start from the bottom? Does this mean I'll only be playing with Bronze players now? Why was my first game with decent rank players and then I have to start at bronze? Thanks
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