You have to spawn a thread using a method of your choice. Once you've done thar though zig gives you some very nice async primitives that let you reenter functions and do scheduling/event loops easily, and it's agnostic; the calls autoconvert to blocking if you don't have async. There was a recent demo of task sharing in zig and comparing to go and rust: (https://youtu.be/UaF6-5BmX2I&t=1h10m)

I've even plugged zig into another highly concurrent vm and gotten it to be a "good citizen": https://youtu.be/l848TOmI6LI (these constructs are running concurrently in test and CI so I know they are robust)