How do I know if my SharePoint site is modern?
If you have an existing site in SharePoint Online, you probably already have modern lists and libraries. If the site doesn't use the Publishing feature (such as a Team site), just create a new page (in the "Site Pages" library) and by default it will be modern.
What is the difference between modern site and classic site in SharePoint?
The “Classic” experience is a highly customizable user interface that integrates SharePoint lists and libraries. The “Modern” experience is mobile ready, (currently) limits the visual customization that can be done, makes the creators' experience more intuitive, and makes the user's experience more collaborative.
Why are there modern sites in SharePoint online?
The modern experience in Microsoft SharePoint is designed to be compelling, flexible, and more performant. The modern experience makes it easier for anyone to create beautiful, dynamic sites and pages that are mobile-ready.