What happens to my character experience if I’ve reached level 20 but my adventure rank is not yet 15 in Genshin Impact?
In Genshin Impact, your character levels (and weapon levels) should be higher than your adventurer rank. Otherwise, you’ll have to spend too much time hacking at monsters.
For example, my adventurer rank is 46, my characters are in the 60–75 range, and enemies levels are in 70–85 range.
When you hit level cap, the characters can’t get any more experience, and any extra experience you’ve fed them goes to waste. You need to do “ascension” to go further. For an ascension, you don’t need any special quests, you just need to gather special materials and money. The ascensions are locked behind adventure level, but it’s very hard to actually hit that cap, as adventure level increases too quickly.
Note that unlike character levels and equipment levels, adventure exp continues to gather even when you’ve hit an adventure level cap and need to do a promotion quest. This means you can gain two world levels at once when you finally do that promotion quest, which will be harsh.