New to the Game and Don't Understand Card Drops

Can someone explain to me how previous seasons' cards and them dropping into the general pool works? Also current season cards being added to the pool? Like I read that they drop into pool 5 at some point, but everything I've read says pool 3, 4, and 5, all start at CL 486. Honestly I'm just very confused about how to get the older additions and brand new stuff outside of the original pool.

Also how /when do cards become available via the Collectors shop, and apparently card prices will decrease over time? Lots of moving pieces here for me lol.

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For context I started playing during towards the end of Modak's season, and am currently around CL 600.