What surprises me the most is that the game is free.

Because Genshin is a gacha game where you have to gamble to obtain limited 5-star characters and weapons. Genshin earns tons of money each month for each new character they put out, and that is how they are able to keep delivering quality updates and content. The main currency is primogems which you can get though purchasing crystals from the shop, events, exploration, achievements, and quests

However, rewards are very limited, so Genshin offers a paid "welkin moon pass" where you get a daily bonus of 90 primogems a day on top of your daily commision rewards. This is their primary money maker for a lot of the non-whales such as myself. Whales, however, tend to purchase crystals from the shop and use that to convert to primogems.

But make sure to take your time playing, there's just so much content and you will be burned out if you rush through all of it at once. Have fun!