Allow CSP Delegated Admins to Manage Modern Sites
As a CSP, receiving delegated admin access from a customer does not allow you to access modern site collections (Communication sites, modern Team Sites).
Delegated admin allows you to access the SharePoint admin centers for both classic and modern sites. In classic, if a site collection has Company Administrator as a site collection admin, a CSP user with delegated admin has rights to the site collection.
In the new SharePoint Admin Center, Company Administrator cannot be added as a site collection admin for modern sites, nor can anyone add my account as a site collection admin because my account is obviously not a licensed user in my customer's tenant.
Per MSFT, this is expected behavior because not even global admins receive access to modern site collections unless they are specifically added by the site owner.
The result is that if my customer wants me to manage their modern SharePoint sites, they must create a licensed user account for me to use. As you can imagine, customers are not eager to burn one of their licenses.