As we know that the people using their mobile devices for searching the content rather than their desktops.
There are two main tasks that search engines perform: indexing and ranking. Firstly, you should understand that how indexing and ranking works. Indexing and ranking are two processes with their purpose and characteristics.
Indexing is the process of adding website or web page to a search engine’s results page and store in its huge database. When the user enter their queries on the search engine then this database is response.
Ranking means the position of the website in the search engine result page and this depends upon various factors like keywords and devices etc. If the website is not indexed the site cannot ranked. Once the keyword is entered into the search box, search engine will check for pages within their index which are closest match.
Google Mobile-first indexing:
Mobile-first indexing means that the mobile version of your website become the starting point for what Google includes in their index, and how they determine rankings.
When your website is the mobile-friendly version then is automatically receive a ranking boost for searchers. When your site is not mobile-friendly version, the desktop site can still be included in the index. but the lack of the mobile friendly experience it could impact negatively on the rankings of the sites.
As we know that today most of the sites using responsive webdesign and provide the mobile friendly version, so the content is same on the both mobile and desktop version and you may not have to do anything differently.
The problem occur when the Mobile site content is less then the desktop site content. So you have have to ensue that the mobile version of your site has enough content to rank your website in the first place. Try to make your website is same like the desktop website. If website is not responsive then you should take care about the following things.
Make sure that the mobile version has all the content, image and video that exists on your desktop site and also the format must be same. You should include the same structured on both the mobile and desktop version and also the meta title and description are same on both version of all pages.
Google is changing the indexes process. but the ranking of the website is still same. As the Google admitted that the Mobile first index is an experiment and depending on what they see after it is fully implemented, they may decide to go back their Desktop first indexing. So you should work to provide the best user experience.