Google Index

In the past, Google, which is the most popular search engine, has had two separate indexes. They started out with their desktop and laptop index.
When mobile started to come onto the scene and smartphones came out,
they created a separate index for mobile. And then, you know, a year or two may have passed. You probably started hearing the term, mobile-first. Mobile-first indexing. What that simply meant was that the search engines were going to start ranking your mobile version over your desktop version.
But they still kept two separate search engines. So if you were looking
at the Internet on a desktop, then you were served a desktop version of Google's Index. If you were using your mobile phone, you would get the mobile version.