Use the .justify-content-*-center to display flex items in the middle of a flex container on different screen sizes.
Resize the browser window to see the effect.
.justify-content-center:
.justify-content-center
.justify-content-sm-center:
.justify-content-sm-center
.justify-content-md-center:
.justify-content-md-center
.justify-content-lg-center:
.justify-content-lg-center
.justify-content-xl-center:
.justify-content-xl-center
.justify-content-xxl-center:
.justify-content-xxl-center