Last year Google introduced Mobile indexing now Google planned to roll out the mobile first indexing on 1st july. This will happen for new website which is responsive means that website which is producing same URL for mobile and desktop.
Mobile indexing means that if there is new website launch than it will be crawled by Google’s Smartphone Googlebot, and its mobile friendly content will be used to index its pages.
You can check mobile indexing with the help of search console tool, When you will login to search console you will see there is a one tool i.e. URL Inspection tool as per below screen.
Now when you click on this tool a search box will appear where you need to use your URL to see how your url is crawled and indexed. For older website Google will continue monitoring and evaluating pages for their readiness for mobile first indexing and will notify through search console once they are seen as being ready.
Same if you will search for old website you will not see this mobile indexing there. So below is the screen when you will search your URL in inspection tool.
One this url will be ready for mobile indexing we will notify by Google.
Now Google suggesting to create new website, which must be responsive so that it will help in mobile first indexing. There are many website which are using different URL for mobile website so now there should be a single URL for mobile and desktop.
If you have any further query or question, you can connect at Google blog or webmaster forum.