CSS Flex Box is the CSS module that allows developers to provide a more flexible layout to the box model, meaning pieces of the box can move around somewhat dynamically (albeit only in one-dimension) and allow for space between each other. The MDN resource provides basic information on what the flex box is, how it works, and the syntax to use it. It also provides demo code to display what the code looks like in use. This is by far the best resource to learn the CSS flex box module and there's plenty of information in the link above.
W3Schools mostly provides the same information regarding the css flexbox module. The only real difference in the page being that the visitor has a lot more demo code sections to view and edit. It's a great resource to find snippets of code to play with and even use on your own page.
The last option is CSS Tricks. On CSS Tricks, there is a comprehensive description of the CSS flex box module, using demo code, illustrations, and a lot of text to explain why CSS flex box works the way it does and how it works. It also further explains the deeper parts of the module, like using the gap property to provide more space between flex content.
To Summarize
Overall, all three resources are adaquate when it comes to providing visitors and developers with information on how to develop and implement certain CSS properties, in this case the flex box, to their own pages. I personally prefer to use MDN and W3Schools as my primary resoruces, but the world of coding and programming is so large that you can find correct information almost anywhere, whether it be from instructors or self-employed developers, information is not hard to find.