If Biden resigns as president, yes Harris immediately becomes president. That means that Speaker of the House Mike Johnson is only one heartbeat from the presidency until January 20.
There’s no way for Harris to appoint and get confirmed a replacement vice president, and it would be impossible anyway because replacing the vacant VP slot requires a majority of both the House and Senate.
It doesn’t when work for becoming the new nominee. Becoming President due to a vacancy has exactly nothing to do with becoming the nominee for the election for president.
Becoming President in this case is by the Constitution. It’s the law, it’s how our government works.
Becoming the nominee is internal political party, legally private non-governmental business.
What does work, whether or not one wants Harris as the new nominee, is this:
Biden drops out as the Democratic nominee for President, while remaining in office as President saying he must put all his energy and attention into governing these remaining months.
Then,
IF the party wants Harris. Harris aggressively campaigns to be President ELSE
Harris states she has to put all her energy and attention into helping Biden govern so will not accept a nomination.
Open convention occurs, but likely with a lot of prior discussion and alignment towards one or two best candidate.