After many years of absence, the father (Skarsgard) returns to the family home after the death of his ex-wife. He intends to reconcile with his daughters and film a screenplay he has written. He wants to cast his youngest (played by Renate Reinsve), an actress, in the lead role. But she, still angry at him, refuses. And so, he turns to a younger star, played by Elle Fanning.
It's a traditional-enough opening for many an interview: "so-and-so can do anything!" But "anything" can carry a lot of water. Harder is actually doing anything, and doing it well. Nia DaCosta can do anything and do it very well indeed - family drama, hard-hitting horror reboot, MCU entry, inventive adaptation - and so it's understandable that she likes to take a little break between all those different things.
Tarantino stated, "I think Inglourious Basterds is my masterpiece but Once Upon a Time in Hollywood is my favourite." This reflects his personal connection to his works.