I concur. I’ve done one or two that are nowhere near as fancy as some that you see. And it really does just take a lot of time. When I did it I did a lot of path offsets and using the different pathfinder to union/subtract/intersect the paths to make them correct for the design.
For instance, see this post about one: Made another stacking mandala today
You can see screenshots of the work that went into the mandala design. There is definitely a lot of work to make them.