Speaking personally, I take my cue from existing cartography experts, like National Geographic. For instance, if you look at a map of the U.S. and you trace the Mississippi river, you'll see it more or less flowing down to the Gulf, north to south, with lots of good little "meanders" or squiggles thrown in. But if you zoom in on sections of the Missouri-Arkansas-Louisiana region by getting another, much more detailed map, you'll see a ridiculous amount of river meandering, with tons of oxbow lakes all over the place, and marshy wetlands where the river once flowed, etc. All that stuff was, of course, not drawn on the big map of the U.S. because you'd never be able to see it at that scale.
As far as I can tell, this is an issue that cartographers have always had to deal with, down through the centuries. You draw the level of detail that is appropriate for the scale of map you're making. You assume, and hope your viewership assumes, that there are lots of smaller details that would be apparent on any zoomed-up map.
Here's the important thing, as far as I'm concerned. When you do the zoomed up map of the smaller region, you can't put in any details that would blatantly contradict the look of the bigger, low-detail map. Sure, you make your zoomed-up river do lots more squiggly meandering, but if your simple river on the zoomed-out map flows south of a particular mountain, you've got to make the zoomed-up river flow in roughly the same course. If the zoomed-out map shows two rivers clearly converging upstream from a particular city, it might be a mistake to then draw the zoomed-up map with those same rivers actually converging within the city walls. That would confuse people looking at both maps. Maybe, when you're working on the zoomed-up map, you utilize a semi-transparent overlay of the zoomed-out map to remind you where everything goes.
The biggest issue for me, personally, is how to represent mountains at the various scales. In fantasy mapping, we tend to draw a lot of isometric-style mountains that are fun and evocative, but which are less realistic than they could be, compared to real-world cartography. If you do a wide map with mountains like that, and then do a zoomed-up region that includes some of those mountains...well, they're going to look silly if you just scale them right up. Do you re-draw the whole mountain range at the new scale? Do you take cues from the zoomed-out map while doing so? It's an artistic problem to solve, for sure.