CSS-TRICKSCSS-TRICKS Website is great site to learn Flexbox. The site has complete guide and explains everything about flexbox.
MDNMozilla Developer Network is another site that works that is usefull resource for Flexbox. It talks about a lot of things such as main axis, flex-direction.
FreeCodeCampFreecodecamp is another website that has all the Flexbox properties to learn. and it is design cool way to comprehend.
In summary the resources are usefull and show great description about CSS Flexbox. They are legit websites that helps any with coding.