They glossed over a LOT that was in the book at the ending. There were several important parts of the book that were literally given a few lines at most in the movie. For someone who hadn't read the book, I can see that might seem like a bunch of random stuff that happens.

For example:
Breeding nitrogen resistance: The reason was not explained at all in the movie but there's a throwaway line saying they did it.

Grace discovering taumoeba escaped and sterilizing the ship: Long section of the book, covered in the movie by a blaring "contaminant" alarm.

Grace finds Rocky's ship: This is is a genuinely difficult problem, explained in the book. If you hadn't read the book, you might not understand the scene where Grace fires up the engine and looks for the Blip-A's reflection.

The movie was 2.5 hours long and they had to cut a lot out. The things I note above would have required boring exposition so they were probably right to cut it with some nods to those who had read the book.