So... how does "My Shop" choose the offers?

This will be my first one (When it is fixed) but I wonder how it's decided. Does it go by x number of plays with that champion or does it go by masteries? In the case of getting a skin for a champion you don't own, does it automatically include that champion? (Let's say Shockblade Zed with Zed?). I'm curious but maybe it's just completely random?
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