Responsive Design
Responsive design serves the same HTML to every device and adapts the layout with CSS. Google recommends it over separate mobile sites or dynamic serving, because one URL and one set of content removes an entire class of indexing problems.
Why it matters for your rankings
Responsive design is the default recommendation because it makes content parity automatic. There is one page, so the mobile and desktop versions cannot drift apart, which is the failure mode that breaks mobile-first indexing.
Separate m-dot sites and dynamic serving both require maintaining two versions in sync, and they never stay in sync. Content, structured data and internal links diverge, and since Google indexes the mobile version, whatever is missing there is missing entirely. Responsive is not automatically fast, though. Serving desktop-sized images to phones and hiding rather than removing heavy components produces a layout that looks correct while performing badly on exactly the devices most of your traffic uses.
How to check it on your site
Test at real breakpoints
Check 390px, 768px and 1280px. Look for horizontal scrolling, overlapping elements and unreadable text.
Verify content parity
Compare visible text, links and structured data between mobile and desktop rendering.
Check images are served responsively
Use srcset so phones download appropriately sized images rather than desktop originals.
Confirm tap targets are usable
Interactive elements need adequate size and spacing, which is both a usability and a ranking consideration.