Hello. In a nutshell, you can’t explain here, so I leave you a link to an article where it is described in great detail about creating a sass grid system https://www.halo-lab.com/blog/implementing-a-grid-system-using-a-simple-hand-written-single-pass-compiler-in-sass. Everything is detailed here, I'm sure you'll understand.

Thank you. You helped me a lot.