Direct import of MS Project files into Planner Premium is not supported. With the retirement
of the Project for the web, the only way to import MS Project files is to use PowerApps or the
Dataverse API.
Also, the default PowerApps functionality only allows importing MPP files one by one; bulk
import is not supported out of the box. It can be done with the API or using PowerShell.
Additionally, the "Advanced Import" functionality allows importing the Project Team and the
resource assignments.