Google's mobile-first indexing has some publishers confused. They see their site ranking at the top of mobile search results for a keyword and a lower ranking for the same keyword on a desktop device. This leads some individuals to believe there are two separate indexes, but there is only one index. Google crawls pages and indexes them, showing users the appropriate mobile or desktop version depending on a user's device.
Have the same information on both your desktop and mobile-friendly versions of your website. If you don't include all your content on the mobile version, Google will use the mobile version to determine ranking and it will lower your ranking. Both sites have to be the same. If you have a responsive website, you don't have to do anything. You are already ready for mobile-first indexing.
Mobile and desktop sites can rank differently. You could use a desktop PC and have your site come up on the first page of the SERPs and when you search for your site on a mobile device, it could rank lower. There are several reasons for this discrepancy, Google ranks slowly loading mobile sites lower because they don't provide a good user experience. Google also gives a lower rank to sites with intrusive ads, especially popups which the visitor has to close before accessing the main content. Smaller popups and banners, especially those concerning cookie policies or age verification, are okay.
Some webmasters are using browsers they are signed-in to check their site's ranking, This skewers the results because, if you sign in and constantly look for a certain keyword and then click on your site, Google will rank it higher for you. The search engine will assume you found the site helpful and increase its rank for you. Always use non-signed in browsers to check your ranking. For more information click here https://www.reddit.com/r/SEO/comments/fo5r4l/pleasehelpforsamekeywordssiteranks/.