How much of her campaign is she really putting together?

A fair amount.

Most of the top echelons of the Biden campaign were Biden loyalists. Those folks are out. Anita Dunn is now Anita Done. Mike Donilon is Mike "Gone"-ilon. Etc. Not just because Harris doesn't want them (she wants her own team), but they probably don't want any part of the campaign either. Their guy is gone. Sure, that doesn't apply to the guy running the phone banks - but it does apply to every single member of campaign leadership, and probably many of the state campaign chairs as well.

Second, she and her new team have to figure out an entirely new campaign strategy. She's not Biden. His strengths are not her strengths, and she's not going to have exactly the same positions or priorities that he had. She can't run a campaign based on, "Look what a great job I did as President during the last four years" - because she hasn't been President the last four years.

It's not at all like switching in a back-up QB. The role of the back-up QB is to learn the existing system and run it if the starter goes down - every other player on the team stays exactly the same, and no past history carries forward into the next play. That's not the role of a new nominee, who will be replacing almost all of the important players, entering into a role she's never had to be in with donors and other party leaders, and changing all the plays.