Password Generator App built with React

I am unsure of how to setup my CSS to make the app more responsive to different device sizes, this includes when to use CSS grid and flexbox and if I should use rem for all the fonts. See the App.css file for all the CSS code. If anyone has good resources or a course that helped them in CSS, i would be interested to check it out!

I am learning React so any feedback on my React code or feedback in general is welcome.

Thank you!