They won't be on the grid for 2 years, and even that is dependent on FoM and the Teams agreeing, but in that time a lot can change. I would imagine they would arrive with drivers from their own academy or from McLaren (Brown and Andretti are close friends and work together in Indicar).
I hope that they they do get a slot, I really do, but I suspect the other teams will stone wall them.
If they do get in then the top F1 drivers will watch with interest, especially with an expanding US fan base and good sponsorship money, but, I think they will need to see that Andretti are in F1 to win and not just prop up the back markers or mid field.
They will get great PR from Danica though, seeing as they were predominant in her own claim to fame.
If they get in and punch above their weight in 2026 even Hamilton or Alonso may be tempted...............