Just to elaborate more, it is an open convention because the delegates are unbound, they could vote for anyone now that Biden has withdrawn. To get on the ballot, anyone just needs to get the signatures of 300 delegates saying they support them. If the vast majority of delegates publicly endorse Harris before the convention though, it’s essentially already over. As this would indicate she would easily win the first ballot (and no, before anyone asks, super delegates cannot vote on the first ballot).