The responsiveness of bootstrap's grid is something that css doesn't have. With a few classes I can make the same site for mobile and ultra wide screen.
You'd do it with media queries and make different grids for different screen sizes. If anything it's more flexible than bootstrap because you can change element order as well.
Bootstrap grid system is super complete, and probably better coded that anything I can make. Its also available as a standalone module, so the size is pretty small. I just wanted it integrated to PicoCSS because I'm lazy.
I usually use https://classless.de because it supports theming, so I can add the bare minimum CSS to make it feel like a different site to another of my sites using it.