For Flexbox https://css-tricks.com/snippets/css/a-guide-to-flexbox/ is the resource I would recommend, along with https://flexboxfroggy.com/ for a more interactive tutorial.

For CSS Grid, the equivalent resources would be https://css-tricks.com/snippets/css/complete-guide-grid/ and https://cssgridgarden.com/

These are all tutorial-level documentation. Once you've gone through those you can complement them with the MDN documentation on the individual properties as a reference.