In mobile-first indexing, Google will only get the information of your site from the mobile version, so make sure Googlebot can see the full content and all resources there. Here are some things to pay attention to:
Robots meta tags on mobile version
You should use the same robots meta tags on the mobile version as those on the desktop version. If you use a different one on the mobile version (such as noindex or nofollow), Google may fail to index or follow links on your page when your site is enabled for mobile-first indexing.
Lazy-loading on mobile version
Lazy-loading is more common on mobile than on desktop, especially for loading images and videos. We recommend following lazy-loading best practices. In particular, avoid lazy-loading your primary content based on user interactions (like swiping, clicking, or typing), because Googlebot won’t trigger these user interactions.
Be aware of what you block
Some resources have different URLs on the mobile version from those on the desktop version, sometimes they are served on different hosts. If you want Google to crawl your URLs, make sure you’re not disallowing crawling of them with your robots.txt file.
For example, blocking the URLs of .css files will prevent Googlebot from rendering your pages correctly, which can harm the ranking of your pages in Search. Similarly, blocking the URLs of images will make these images disappear from Google Images.