Placements in League

Hello. Can anybody tell me how it works? I play games always with golds, plats or diamonds, so after few years i go play ranked, and after one game (win) im in bronze II 74 LP. In that game everybody had gold. Every game have deiffrent placement or now I will go from bronze to silver? Don't know what's going on. Few years back when u play ranked, you had to play 10 games and then you've got your placement, now after 1 win i get bronze ^.-
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