It might seem silly but there a few games you can play to learn flexbox or grid. For example:

Grid Critters: https://gridcritters.com/

Flexbox Zombies: https://geddski.teachable.com/p/flexbox-zombies

Flexbox froggy: https://flexboxfroggy.com/

I think you can see these games as pieces of a puzzle. You will need all the pieces to understand the big picture.

Also there a some books from A Book Apart you can try. https://abookapart.com/search?type=product&q=CSS

Maybe start with CSS3 for Web designers by Dan Cederholm https://abookapart.com/products/css3-for-web-designers

Another suggestion is to subscribe to The Codepen Spark: https://codepen.io/spark/