Base Apparel CSS, HTML Responsive Design with a spot of JS

-I ended up removing the footer for the desktop version b/c I couldn't get it to the correct position... :/ -Many things I can improve I know, but I am taking a break from this project. -I'm confused about the difference between Grid and Flexbox, and when to use which. Can I use them both in the same website? I gravitate towards Flexbox because I know a little bit about it.