Answers for "center single child of flexbox"

flexbox align or justify one single item

margin-right: auto; /* align left ⇦ */
margin-left: auto; /* align right ⇨ */

margin-bottom: auto; /* align top ⇧ */
margin-top: auto; /* align bottom ⇩ */

flex align children to side

flex-direction: column;
align-items: flex-start; //left
align-items: center; //center
align-items: flex-end; //right