Selene Tries And Fails To Save Her Dreams And Her Family

One of the lingering questions at the end of the game is answered in the true ending. The astronaut is Selene herself.

Is Selene the child Returnal?

This ending (and the house sequence events that precede it) also seem to confirm that Selene wasn't the one driving the car in the original ending and that she was the child. The real driver was her mom, Theia.

Is Returnal all in Selenes head?

The simple explanation is that all of Returnal is taking place in Selene's head. The White Shadow signal she's been chasing? That was simply the light above the water's surface which her car crashed into.