Responsive design means one website that reshapes itself to any screen. And it fully replaced the separate 'm.yoursite.com' mobile-site era. If anyone proposes building you a separate mobile website in 2026, leave politely.
You almost certainly have responsive design already (every modern builder and theme ships it). The real question is whether it's GOOD on mobile: readable without zooming, tappable buttons, a phone number that dials, forms that don't fight autocomplete. Checked on your actual phone, not a resized desktop window.
Why it's non-negotiable: mobile is where most local searches happen and Google indexes the mobile version of your site first. Your desktop site is the side view; the phone is the storefront.