Bootstrap Grid classes in relation with device width

.col-xs- (extra small devices – screen width less than 576px)
.col-sm- (small devices – screen width equal to or greater than 576px)
.col-md- (medium devices – screen width equal to or greater than 768px)
.col-lg- (large devices – screen width equal to or greater than 992px)
.col-xl- (xlarge devices – screen width equal to or greater than 1200px)

Each class scales up, so if you wish to set the same widths for sm and md, you only need to specify sm.

Gutter width 30px (15px on each side of a column)

References
https://www.w3schools.com/bootstrap4/bootstrap_grid_system.asp
https://getbootstrap.com/docs/4.3/layout/grid/