When I create a new SPO site collection, the site by default appears to be a classic team site. I want the top level site to be a modern site, Can i create top site collection as a hub site?
Yes, you can add a root site as Hub in SPO. A classic site collection can be a hub site. Sub site can't be added to hub and the subsite themselves cant be a hub site. Only a Site Collection can be a hub site.
Also, only the modern pages in Site pages library of the classic site collection will have the benefits of the Hub site.
So, you need to enable the Site Pages feature to have modern site pages and the benefits of Hub site. It is site level feature (
Site Settings > Manage Site features)
To add it as hub site, use below PS:
Register-SPOHubSite https://tenant.sharepoint.com Set-SPOHubSite https://tenant.sharepoint.com -Title "Tenant"