Use .flex-column to display the flex items vertically (on top of each other).
Use .flex-sm|md|lg|xl-column to achieve this on different screen sizes.
.flex-column:
.flex-column
.flex-sm-column:
.flex-sm-column
.flex-md-column:
.flex-md-column
.flex-lg-column:
.flex-lg-column
.flex-xl-column:
.flex-xl-column
.flex-xxl-column:
.flex-xxl-column