https://web.dev/learn/css/flexbox/ is a resource that teaches flex in depth, and has visual, live examples of coding that you can play around with. The article does a good job of explaining how to use flex, without overwhelming the reader. It gives a pretty decent overview on the subject.
https://flexboxfroggy.com/ is a unique online game that teaches how to utilize flex in CSS. The game helps users understand how to use flex-direction, align-items, align-content and justify-content on parent containers to align the child elements appropriately. It also teaches how to order things on individual child elements. It is a more interactive approach to learning how to utilize flex properties.
https://flexbox.malven.co/ is another resource to learn flex concepts. I like how simply the page provides you with the information, with images of the results of using certain properties. It also seperates properties depending on if they're used on the parent or child container.
Overall, there are several resources on the internet for learning flexbox properties and values for both parent and child elements. Some resources focus on a very descriptive and detailed explanation of flex, while others focus on a more interactive and visual approach.