If those events happened, then the Speaker of the House becomes president.
No, I don’t think there has ever been a time in history, where both the presidency and vice presidency are vacant at the same time.
So if Harris is gone, the office unfilled, and Joe Biden suddenly died, then the Speaker becomes president.
Obviously dems are aware of that, so we would expect them to swear in the replacement forthwith if Harris resigned. Another possibility is, Biden resigns first and Harris promises to do the same after the next VP is sworn in. Of course a few fanatics think Harris needs to be the next POTUS but the real rulers would not allow it unless they wanted more chaos.