This page of the Genshin Impactgame guide explains how to unlock the Hangout Event and what this event is for.
The Hangout Event is a series of quests tied to the characters from the Genshin Impact game. To unlock access to this event, you will need to reach Adventure Rank 26. Additionally, to unlock access to this quest, you must sacrifice 2 Story Keys, which can be earned by completing Daily Commissions. If you meet the conditions, go to the Quests tab and then select Unlocking Quests. A window will appear as in the screenshot.
From there, choose which quest you want to start first. You can complete all of them as long as you have enough Story Keys. The quests themselves mainly consist of going out with a given character and talking to them. Depending on the dialogue options you select, you may get different endings. Each unlocked ending gives you Adventure EXP and various materials. You can even get a small amount of Primogems, which are necessary to obtain characters and items.