You can start from the chapter that picks up right where the movie ends. Usually, you can find this information in fan discussions or on manga-related forums.
The Jujutsu Kaisen anime has covered certain key arcs from the manga, but it's not caught up completely. You'll have to read the manga for the full story.
It leaves off at a specific point that ties into the main manga storyline. But to be precise, you'd have to check the chapter numbers for a clear understanding.
The Jujutsu Kaisen manga is indeed ending. It's been a great run, but all good things must come to an end. The final chapters are bringing closure to the characters and story arcs.