In Sumeru you can find an area called Vanarana, where you will find the doors to Dendro's sphere, so it will be necessary to complete the quest to unlock Vanarana first.
The first thing you have to do to access the Vanarana quest, is to find an NPC called Rana, with whom you will have to interact to access the Woodland Encounter Quest. After completing the quest, you will have to access the Vimana Agama quest line by talking to Ararycan.
In the quest you will have to find the Secret Base and at one point in the quest you will enter a scene, after which Vamadha's Rhythm, which functions as an exploration tool, will be unlocked.
When you reach a series of rocks you will be able to move them using your powers, for this you will have to interact with them by activating Music Challenge. Here you will find the solution for the notes you need to follow to unlock the doors of the Dendro sphere: F, E, E, D, D, C and you will be ready to go through the doors in Sumeru.
That's all you need to know about unlocking the Dendro sphere doors in Genshin Impact, so now that we're done, we hope we've been as helpful as possible and we hope you manage to unlock all the sphere doors quickly.