CSS-Tricks seems to have a good amount of resources to use that can teach you in-depth about the use and how to for display flex, there is even some nice illustrations that show you how the code will be used.
Like CSS-Tricks MDN has a very in depth model to show on how to use flexbox and the benefits and uses. While CSS-Tricks had illustrations on how the code is used and what it does, MDN has live examples which is helpful for practice.
While not having as much of an in-depth explanation W3Schools has a fairly immersive practice course for flexbox, they do have a more in-depth explanation but not within the learn hands on part.
In summary, whether you use any of these websites to learn or even check over your code. There is no wrong site to use, they all have a good explanation on how to implement this code and even explains how it works and what it can do to benefit your site.