Yes, it is free-to-play on all the different devices it's available. Genshin Impact supports crossplay, as mentioned above. And despite it being an online game, you don’t need a PS Plus subscription in order to enjoy it on the PS4 and the PS5.
Considering how large the game is, it’s quite awesome that it can be played completely for free if one wishes to. However, this cross play enabled offering does have an in-game currency system that’s powered by real money transactions.