Why does Bootstrap 4 distribute col like this?

Question

css

I haven’t made up with Bootstrap for a long time , apparently, I forgot something. In Bootstrap 4 , if I understand correctly, there is no need to specify a window width reduction for devices like xs, sm, lg ...
I split the row into 2 parts using col-6 , but somehow it does not work. How can I fix it so that at any screen resolution I have blocks on the left and right of the same width, as if I had given them width: 50%?
Example

Answer 1

The link doesn't work because Bootstrap is crookedly connected. There is no error in the layout.

Answer 2

Bootstrap 4 is powered by flex. Know what it is?