freeCodeCamp: Flexbox Explained
freeCodeCamp explains the difference between the flex container and flex items, as well as the properties that are associated with each. It also shows how to center flex items within its container.
ionos.com
This page shows how some flex items can be different than others while still behaving as you would expect within its container.
Josh W Comeau
Josh takes a look at some of the default behaviors of flex containers and items. It also has some interactive sections that explain how different properties work.
Flex Summary
Flexbox is a method of laying out elements on a webpage that is an upgrade over floats, which is what was used before flexbox. It is optimized for laying out elements in one direction, either as a row or a column.