How do Pokémon collectors find cards?

It might be a dumb question but I have no idea how the value or collecting even begins.
My boyfriend recently got into collecting Pokémon cards and his birthday is coming up.
I notice people sell their cards online and the prices vary. So I just wonder, do they stumble upon these cards in a pack they buy at the store? Is it a luck of the draw type of ordeal? Or do they specifically buy limited edition cards and resell them? I’m just wondering if he would like a pack of cards to test out his luck like a lottery ticket or if he would enjoy me specifically finding something that has been valued.