The PS4 edition of Undertale with the dog shrine has reminded me of a headcanon that I’ve not posted about yet. Basically it’s to do with space and shortcuts.
The new thing that’s prompted this is the dog shrine behind the skeletons’ kitchen. Now physically that just totally does not make sense, right? I mean, there’s not literally a cavern behind Papyrus’s sink (though it could be under the kitchen floor, I suppose).
So my answer for when space doesn’t make sense like this: shortcuts in the doorways. In this case, there would be a portal at the back of the cupboard that leads to the dog shrine cavern elsewhere in Snowdin. So you could walk all the way around the house, dig beneath it, everything… and never find the cavern. Physically it’s located miles away in an unknown location.
Where this gets into more headcanon territory is applying similar logic to the Core, and considering how the portals are made.