How is the current election campaign going to work with both Harris and Biden as sitting senators? And if they win the general election, what happens to their seats? Do those senate seats get appointed in a Democrat Primary or is it a general election in those states?
Biden is not a sitting Senator, his last political gig was VP. Kamala Harris is a Senator from California, so her seat is safely blue.
ETA: I see you are from Australia, so I will explain a bit further wrt Harris. In the US, the State Governor is responsible for choosing who will replace a Senator who cannot complete their term. In California the Governor is a Democrat, so he would pick a another Democrat.