Almost half of the land (46%) in California belongs to all of us - the citizens of the USA.
The Calexit people should leave the USA if they want to but the land stays with us right here in the USA.
They could buy it from the US fed government. It would only cost them a couple trillion dollars for starters.
Here’s a link to the federal lands and indian reservations within California.
https://nationalmap.gov/small_scale/printable/images/pdf/fedlands/CA.pdf
None of the federal lands will be part of any secession. I have no idea of the BIA has any say in what the tribes can or cannot do vis a vis joining a secession movement or not.