The way I understand it is that the state members in the House gather together by state and vote.
For example New York has more Democrat House members than Republican ones, so when they vote it will be for Biden.
Texas has more Republican House members, so they will vote Trump.
There are more Republican states than Democrat ones in this scenario, so Trump wins assuming no defections.
True, the Repubs would have to hold steady and stay in line with their vot.