The main difference is in how the votes are counted.
In RCV it's a series of tiny first past the post elections all rolled into one. You vote for person A at the expense of person B. You have to choose one or the other, and that forced choice often comes back to hurt you. It's an Ordinal voting system.
STAR is a Cardinal voting system at its core. Think of it this way, giving a 5-star review to the local steakhouse has nothing to do with the 3-star review you gave to the sandwich shop down the road.
Basically, Ordinal systems fall prey to Arrow's Theorem. This boils down to eventually being forced to choose the lesser of two evils. Which then leads to two-party dominance. Cardinal systems sidestep that completely.
This live stream is about three hours long, but breaks down STAR and RCV, and the massive flaws of RCV.