How to make sure that there is no joint?

Question

Adobe Illustrator

If you create 2 objects (in my case, a rectangle) and put them side by side (back to back), and then open the file somewhere else, then a gap (I'm shocked) is formed between them. I am very perplexed.
illustrator:

photoshop:

Video:
https://yadi.sk/i/X5wEqG0f4yAZNg

In case someone tells me about raster and vector:
The same is true when saving to svg. If you save it in svg, and then paste it into the browser (for example), then there will also be a gap.

Not along the guide
in the video, I aligned the objects along the guide. Even if you align them between each other, the gap will still be.

Answer 1

For web and raster - pixel grid alignment. For printing don't care.
It also infuriates me, so I don’t make such objects butt-to-butt, but put them on top of each other, if necessary.