Is Huayan, Chan, and Pure Land Buddhism the only sects that teach about multiverses? The Avataṃsaka Sūtra describes a cosmos of infinite universes upon universes, mutually containing one another. I know this scripture is taught in Huayan, Chan, and Pure Land sects, but are there any other Buddhist sects that teach this?
The Avatamsaka Sutra and these sects teach that infinite universes are connected to a jeweled flower garland or net, and universes reflect each other.
Theravada teacher 31 planes of existence. Then there are many other (millions) similar planes.
https://www.accesstoinsight.org/ptf/dhamma/sagga/loka.html