This grid is taken from Bootstrap 4 and removed the unused unnecessary code.

12 Columns Phone Grid

The grid below is a demonstration of how to use it. If you are familiar with grids It won't be a problem but If you don't you better learn this simple concept because It is around the web for many years.

For Tablets

If you check below columns with your tablet and phone you will see difference. On tablet these columns are near each other. On phone under each other.

You can change number from 1-12 on all column types, like you can create 1 row of 12 columns on tablet but it may show 6 rows of 2 columns on phones or vice versa.