I am F2P(which means no money spent at all) and I have a total of...8 5*s(6 characters and 2 weapons). I've been playing since a week after Genshin released and I'm just going to say that the game is still playable if you are F2P.
The full definition of F2P is a person who spends absolutely no money on a game. In Genshin's case, that means no genesis crystals, so no crystal packs, no Welkin, no BP, no adventurer's bundles, and no skins that cost irl money/genesis crystals.