Inaccessible areas in games, and the could-be life within them

Watching UNDERTALE's 10th anniversary stream, and getting to see Toby's "What if"s of UNDERTALE and it's world...It sparks something within me.

When a videogame world is well-crafted and you get immersed in it, you can't help but wonder how life is to those who inhabit that world. What lies beyond every locked door and every fake building, wondering what it is that occupies its interior.

I have always– since little, loved seeing cut and unfinished content. Seeing an "expansion" of the game I see myself in, the "hidden bits".
It is a strange feeling to put into words, but knowing that there is world out there that is just beyond what you can see and interact with,, it makes it feel like the area you are restricted to is but a piece of this grand world and there is so much more to it than you know. It makes every interaction feel so much more alive, so much more...real..

Something similar happens when an old game gets found out to have hidden content that managed to stay undiscovered for years. Something the world was hiding from you, not because of developer intention, but because the player simply had no motive to search for something that to them, didn't even exist. IT'S JUST SO COOL! Ive always had a heart for stories about players finding some obscure thing that no one else has been able to replicate. It makes the game's universe feel REAL! And I believe it's what can make a game so special to so many people.

Using UNDERTALE as an example for this, you are given a sense that the Underground is a very big place, yet, you only get to explore what is probably a fraction of the true scale of that place. You are given hints at how life is for those you do not meet, but ultimately it is up to the player to imagine it. And in immersing someone to that point, I find beauty...
It is in today's (and yesterday's) streams that a peek into those lives was found. And even though it was a small peek, it was sufficient to spark wonder in many people I hope. In a way telling the player (or viewer, in this instance) that: "You may think you know everything about the Underground, but in reality, you only know what you have played through. You have not truly been to the Underground".

As you may tell, this is something that excites me very much, as well as something that inspires me a lot when creating.
This is my first post here so I'm likely all over the place, but regardless! If you have any similar feelings or experiences about the matter do feel free to share! I would love to read them :3