Genshin Impact, like many other games, contains a Battle Pass. Unlike other games, you actually need to do quite a bit of playing and grinding to get access to it. You cannot just buy the Battle Pass on that hyped up first day of playing the game.
To get access to the Battle Pass, you need to get to Adventure Rank 20. Unlock the Battle Pass gives you access to daily, weekly and seasonal tasks to complete. Doing so will earn you Battle Pass XP which will then rank up the Battle Pass, netting you all sorts of rewards.
The Battle Pass contains 30 levels, and it takes 100 XP to jump from one level to the next and get your reward. Like many other games, the Battle Pass is split into two tiers, the free tier which is called Sojourner’s Battle Pass, and the premium tier which is called the Gnostic’s Battle Pass.
Owners of the premium tier will basically be getting rewards from both tiers as they jump up in levels, and the premium tier costs $20 per season. The current season is set to end in about 40 days, you will have plenty of time to hit Adventure Rank 20 and rank up the battle pass fully before the season ends.
When you have access to the Battle Pass, if you want to upgrade to the premium tier you can just click on the button in the bottom right corner that says “Unlock Gnostic Hymn” to purchase it through the relevant store on your platform.