Flexbox Froggy
This site offers an interactive game based way of learning flexbox. As the user progresses the content that they are learning becomes exponentially more complex until the game ends at level 24 and all the areas of flex box have been explored and mastered.
CSS Tricks
This site offers a side by side view of the coding on the parent container and then the coding for the chilren of that container. It is a nice way of seeing the relationship between the flex container and the flex items.
WEB.DEV
This site offers an interactive space that allows the user to toggle prodperties and the see directly how the items act with a certain value given. It is a good way to conceptualize how items will be affected by the different values.
Summary
There are numerous sites offering information about flex box. The developer just needs to find the site that best fits their learning style. For example, Froggy might be a great way to start learning but may not be practical when looking for a specific piece of information while coding. WEB.DEV might be a better choice for something like that. Where as CSS Tricks might be good when thinking about the relationship of flex items to the parent container.