I know others have already mentioned it, but i've recommended https://flexboxfroggy.com/ to others before and they quickly picked it up.
This is another good one for learning css grid https://cssgridgarden.com/
This is another good one for learning css grid https://cssgridgarden.com/
A useful tool for generating Bootstrap code is https://build.layoutit.com/ and for CSS grid https://grid.layoutit.com/. Unfortunately the former is limited to Bootstrap 4, not 5.