Hi @Vinod Survase ,
If you return to classic, the main features you will lose are the integration with Microsoft 365 groups, new modern web parts that don't require coding, more lists and libraries, a responsive out-of-the-box mobile experience, a Recycle Bin, modern search functionality, and an out-of-the-box Sharepoint news feed. Many of these differences are highlighted in these articles:
https://techcommunity.microsoft.com/t5/microsoft-sharepoint-blog/what-is-modern-sharepoint-and-why-should-i-care/ba-p/161941You should be able to revert to Classic by following the process in the article and updating the settings in the admin center. You can change the look under the Sharepoint settings.
Let me know if this addresses your question or if you have any other specific questions.
If the information helped you, please Accept the answer. This will help us as well as others in the community who may be researching similar questions.