Correct.

California can appoint an interim Senator (per their constitutional & laws - I believe the governor does it without any approval from other parties).

Committee appointment is by Senate rules. There's no automatic right to a seat on a committee. Judiciary is a massively important committee because it oversees the approval of federal judges (without committee approval, the nominees never get to the floor for a vote).

The GOP can block a committee replacement via filibuster. The Democrats could attempt a rules package to prevent filibuster, but Manchin (D-WV) has historically been against those.