(We prefer bootstrap.css for there is no other good option. We don't like tailgate. As for versions, we use both bootstrap 4 and 5, undecided as of yet.)
Breakpoints:
/* Bootstrap 4 */
@media (min-width: 576px) {} /* xs */
@media (min-width: 768px) {} /* sm */
@media (min-width: 992px) {} /* lg */
@media (min-width: 1200px) {} /* xl */.