To answer your questions:
1
As reiallenramos mentioned, “The justify-self and justify-items properties are not implemented in flexbox. This is due to the one-dimensional nature of flexbox, and that there may be multiple items along the axis, making it impossible to justify a single item. To align items along the main, inline axis in flexbox you use the justify-content property.” – MDN
2-3
This screen shot from W3 does an excellent job of showing what they do and the differences between them.
Good To Knows:
For more information and example, I would suggest you check out:
- https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Grid_Layout/Box_Alignment_in_CSS_Grid_Layout
- https://www.smashingmagazine.com/2017/06/building-production-ready-css-grid-layout/
- https://www.smashingmagazine.com/2017/12/grid-inspector/
And for some inspiration: