SpaceX is a private company. Tesla is public, but the board is not independent, still trying to give Musk a pay package of $50 B.
CEO answer to the board, but a board can be controlled by the founder in one way or another. For example, Zuck control shares with more votes and Bezos have vote rights over his ex-wife shares. The CEO can also fill the board with loyalists, as Jobs did on his return.