As per the google webmasters latest update today 28 May, mobile first indexing will take place by default w.e.f. 1st july 19.
According to the news, they have done the analysis of most of the websites and found that websites are ready to for this mode of crawling.
As per their study, now websites are showing the same content on mobile and desktop devices.
You can check your websites to check for mobile first indexing by using the URL Inspection Tool.
According to the update you will easily see your website that how it was previously crawled and indexed?
However, for older websites, the google will continue to monitor and evaluating their webpages for their willingness for mobile first indexing, and they will notify them through search console, once they will be ready to be indexed.
As most of the users are depend on their mobile phones rather PC’s. hence google have now focused to deliver the best experience for mobile users and to serve the best out of it, they have rolled out the mobile first indexing concept on the ground.