Pick a breakpoint system (mobile-first or common device widths), and the tool generates the matching @media query blocks with your target widths filled in.
Mobile-first (min-width queries) is the modern default and generally produces smaller, simpler CSS — pick desktop-first only if your codebase already follows that pattern.
Yes. The output is plain CSS — add it to any stylesheet or CSS-in-JS setup that accepts raw CSS.
Only where selectors or property names match. Check specificity and load order if you see conflicts with existing rules.