Responsive design using CSS flexbox and JS for email verification

Hello everyone. This is one of my first projects involving HTML, CSS and JS. I felt relatively comfortable with HTML and CSS, I think I need to know how to use responsive design, I'm a bit lost in the way it works. Regarding the way to do email verification, in many places I researched they use ReGex, which I'm not familiar with, but I ended up using it. If you have any comments or how I can improve this project, please let me know. PS: English is not my language, I do my best to try to be communicative.