It’s from the 12th amendment. A state elector must vote for at least one candidate from a state other than their own. So electors from Nevada could place both their Presidential and Vice Presidential electoral votes with an all-California ticket but California electors may not. There was talk of Kamala picking Newsom for VP but she’d have to change her residency for that to happen. Everyone would know that would be BS and possibly cost her California’s electoral votes or her candidacy in the second state, so she picked Walz instead (not the only reason but you get the idea). Kennedy got caught pulling a fast one so here we are.