The Earth is not large enough to build a particle collider big enough to test if String Theory describes the most elemental particles or if there is something smaller. We might be only a handful of generations of particle accelerators until spaced based colliders. All of this is to say, you’re going to need to be okay with you personally never knowing this answer. It’s all theoretical.
I cannot comprehend what would be inside a particle that would be a smallest possible one. Division to infinity feels calming.
These particles don't have a volume; they're point-like. They don't exactly have an inside with other particles waiting to burst out. These are just mathematical abstractions after all, good at making some predictions. May or may not be actual reality.
But when I picture a point, I can see it and since it has a visible area, then what's in it? That's my problem. I could picture it as nothing maybe? But then what's in the blank space? It has a volume...
That's more an issue with your mental model. These aren't points, they are fields with potentials.
Probably because we learn about points by drawing them on a graph at some point in math class, so we imagine them having an area. I think points are an abstract construct. Sort of like infinity - you can never truly reach it. Or imagine it.
Seems to me that the way the universe prevents naked quarks from existing blocks particles from getting any smaller.