The Red Panda Realm is a spiritual plane of existence in the film Turning Red. It is where the female descendants of Sun Yee go to relinquish their Red Panda powers after being blessed with it when they come at a certain age. It can only be accessed through a ritual on the night of a yearly Red Moon Eclipse. Sun Yee presides over the place.
The way to remove the Red Panda form in the Realm is to enter a portal to a dark plane of existence that Sun Yee's spirit opens while the Red Panda spirit leaves and enters an inanimate object in the mortal realm where the previous bearer of the spirit will possess. It is stated that if the ritual fails after the Red Moon Eclipse ends, the person who does not have their Red Panda sealed will be bound to it permanently. If the ones who had their panda spirit sealed get their seals broken, they are capable of re-sealing it on the Red Moon Eclipse. The ones who have gained the Red Panda powers have also gained red hair (coinciding with the color of the panda), which replaces the hair color they were born with (in the case of Mei and her family, they were all born with black hair). The Astral Plane also seems to have the ability to regress who was ever blessed with their Red Panda form to their youth as Mei saw that her mother Ming as a teenager during her second visit.
The Realm was first seen in the third half of the movie where Mei enters to remove her Red Panda form on May 25th, 2002. She at first tries to do so, but soon discovers how much joy it brought to her life, most notably from her friends Miriam Mendelsohn, Priya Mangal and Abby Park. She then decides to keep her Red Panda form, abandoning the ritual to attend the 4*Town Concert with her friends. Unknown to her, she damages her mother's talisman containing the spirit of her own panda in the process and Ming soon bonds with her panda form, which is soon revealed to be very giant, in order to retrieve Mei by force.
The Realm is next seen when Mei and her mother, grandmother and extended female family have entered after managing to incapacitate Ming. They soon decide to leave the Astral Realm and retain being human with the exception of Mei, who has decided to keep her panda form for all time. After her relatives leave, Sun Yee arrives. When Mei expresses concerns about her choice, Sun Yee just smiles in pride as she takes Mei with her to fly up to the top of the Realm when both become Red Pandas to take her back to the mortal realm.
The Red Panda Realm takes on the appearance of a Chinese bamboo forest, which is sort of fitting since bamboo is the primary delicacy of a panda. The Red Moon Eclipse can also be seen within the forest, most likely to let whoever is trying to relinquish their panda spirit know how much time they have left to do so.
It's likely the Realm was created by the Gods who blessed Sun Yee with the Red Panda ability.