CSS Tricks
CSS Tricks is a website with very clear graphics and steps showing how to use the "display: flex;" property. This website is a great place to start if you're just starting to become familiar with this property and are still learning about it.
W3Schools
W3Schools is a good website to learn more about the "display: flex;" property if you want to experiment with it and try it out easily. W3Schools has text boxes where you can write, edit, and run code to experiment with various properties.
Mozilla
Mozilla (MDN) is great resource to learn more about anything related to the "display: flex;" property. It has clear graphics which explain the property, but also has areas to write, edit, and run code to experiment.
Summary
Overall, the first website I would consult for help if needed would be Mozilla since it has clear graphics and areas to experiment with code. However, W3Schools and CSS Tricks also have helpful information too.