How did it work? As I know DOS wasn't even a proper operating system, there were no separate processes, only single running task. So, how was it possible for multiple users to run multiple programs in parallel?
You are describing MS-DOS. As article states, this used DRI's Concurrent DOS, which was basically rebranded MP/M-86 with a basic MS-DOS compatibility layer.
More about Concurrent DOS, eg here: https://nemanjatrifunovic.substack.com/i/196574463/quest-for...