The Genshin Impact rummy 365 Udumbara Pistils are a new special quest item hidden inside flowers in update 3.6's new desert region of Sumeru. You'll have to collect these six pistils as part of the Pale Fire world quest that you get from Zurvan after you arrive at the Vourukasha Oasis. While they aren't too hard to find, it can be a little confusing working out how to make the rummy 51 flowers appear.
The answer is using your newly acquired gadget-cross-comparison, Sorush. Here, I'll explain where to find the six Udumbara Pistils and how to use Sorush to actually make the flowers appear so you can grab them.
How to get the Udumbara Pistils
After you go to Vourukasha Oasis and complete the As Part of the Splendorous Sky quest upon entering the new region, you can talk to Zurvan, who'll give you the Pale Fire quest and ask you to find a Fravashi Tree. Head down into the nearby Asipattravana Swamp and you can find one there—it'll also be marked on your minimap with a little circular tree symbol. You'll need to use Sorush to interact with the tree, so equip her as a gadget in the menu, press Z to pilot her, and then hold E to activate her ability by the Fravashi Tree. This will let you talk to the Residual Pari inside, who'll ask you to locate six Udumbara Pistils.
Luckily, these are all marked on the official miHoYo interactive map and can be found in the swamp and on the cliff walls around it. To access an Udumbara Pistil, you first need to make its flower appear by using Sorush. You can recognise where the Udumbara flowers are hidden since they are marked with a little green heart-shaped symbol. Activate Sorush with Z, have her fly up to the heart, and hold rummy best E to keep her in place while you run over. Now, the flower will have appeared and you can climb up onto it to grab the Pistil. When you have all six, head back to the Residual Pari in the swamp and hand them over to complete this part of the quest.