Motion picture film is scanned in with a scanner that works in a similar way to a consumer flat bed scanner
Good scanners are now 8-10k resolution, and in the processing phase at the studio it is down converted to 4K for playback.
Sorry, for those paying attention, I'll flesh this out further.
From the beginning of time, until about a decade ago, movies were captured, edited and played back on motion picture film. 24 frames per second of film was shone onto the cinema screen, each frame doubled exposed, and your eyes and brains merged the 48 images into motion. Completely analog.
Along came "digital". Initially quite average. First only used for cameras at the capture stage, then soon used for editing in digital domain, then (while still printed on film for playback), until finally the playback was also digital.
There are scanners that will take an old film negative (or positive) and scan each frame in individually at a high resolution, usually 6k (6144 x 3240 pixels) and then after restoration/retouching/manipulation is down sampled to 4096 x 2160 pixels for playback on a modern digital projector.
Done right, it can look awesome. Done wrong and YouTube will call wanting its video returned from 2001.
In the case of 2001: A space odyssey, a master copy of the print was found, scanned in and cleaned up, and released for limited theatrical playback, with a 4K blueray release planned soon. It's an expensive process, reserved for worthy movies.