In March Google started moving websites to mobile-first indexing: it now looks at the mobile version of your pages to decide what they are about and how they rank. Most people search on their phone, and Google wants its results to match what they see.
What it means for you depends on how your site is built:
- One responsive site that adapts to the screen: nothing changes. Same pages, same addresses, same sitemap.
- A separate mobile site, like m.example.com: make sure the mobile pages have the same content and the same links as the desktop pages. A page that only the desktop menu links to may be missed.
Your sitemap keeps listing the pages of your main site. Search engines use it to find pages that are hard to reach through links, and that matters even more when a mobile menu shows fewer links than the desktop one.
Not sure how your site looks on a phone? Google's Mobile-Friendly Test checks a page for you.