Chris Coyier's guide A Complete Guide to Flexbox on CSS-Tricks.com is a very useful guide on how to use flex layout, with a clear separation between properties for the parent(s) (flex container) and children (flex items). The visual representations of how flex works and the provided code snippets do a great job of demonstrating how the different properties affect content. The examples and related articles at the bottom of the page also provide useful information and demonstrations on different kinds of styling that can be achieved using flexbox.
The page Basic concepts of flexbox on MDN Web Docs contains a great overview of how to use flexbox to achieve basic styling designs, with the added benefit of interactive code snippets to let the user see flex in action. At the end of the article is a link to a page titled Relationship of flexbox to other layout methods which provides further information on how to use flex in conjunction with other layout styling methods.
Benjamin Semah's article The CSS Flexbox Handbook - Complete Guide with Practical Examples provides a thorough overview of what flexbox is, its benefits, how the layout is determined, flex containers and flex items, the difference between flex and inline-flex, and all the different properties that can be applied using flex layout, as well as ample visual examples and premade practice modules. There are also links to a few flexbox-based games for further practice and a brief rundown of some of the bugs that may be encountered when using flex layout.
The three articles above all provide the same information, but the differences in their presentation make them all valuable resources in their own right. CSS-Tricks has been my go-to for refreshers on how to use flex layout and I appreciate the split between parent and child properties so as not to get them mixed up, but I also appreciate the interactive code on MDN Docs and freeCodeCamp, and the flex games are pretty fun while still being helpful and informative. I'm sure I will be referring back to these resources quite often.