In Genshin Impact, you’re going to need to manually level up your characters to make them stronger.
While characters do gain EXP from killing enemies, the amount they gain from this is extremely meager. Instead, you’ll want to use EXP material you find around Teyvat: Wanderer’s Advice (worth 1,000 EXP), Adventurer’s Experience (worth 5,000 EXP), and Hero’s Wit (worth 20,000 EXP). Meanwhile, killing a monster gives about 15 EXP, so the difference is enormous.
These three EXP materials can be found in treasure chests and they’re often given as rewards for finishing Domains and quests.
You can also farm Adventurer’s Experience by activating the Blossom of Revelation Ley Line Outcrops found around the map. These blue smokey balls are visible from your map and spawn in various locations. You can activate these, and they’ll spawn enemies to take down. Once you finish them off, you’ll be able to claim a reward of around 13 Adventurer’s Experience in exchange for 20 Resin.
Once you have your materials, you can hit the “level up” button on the character screen to use them. Once they hit certain level milestones, they’ll need to be ascended using materials gathered from bosses in order to level up higher.
Keep in mind that using these materials to level up your characters require Mora, Genshin’s name for all-purpose coinage. Leveling up tons of characters to high levels does eat up lots of Mora, but you can grab more by taking on Blossom of Wealth Ley Line Outcrops, which spawn enemies for you to defeat.