There are plenty of tutorials and articles to learn the basics of CSS. But a very few to understand concepts like positioning, flex, margin, etc.
So, here are the 3 resources that will help you to understand confusing CSS concepts.
Learn Layout- This is an excellent website to learn concepts like margin, box model, float, inline-block, etc.
Ishadeed- This blog visually explains the confusing CSS positioning concepts like relative, fixed, absolute, etc.
Josh Comeau- This blog explains CSS concepts like z-index, animations, etc.
I highly recommend beginners to visit these blogs so that you'll love CSS forever!