For Sisense, it stores its meta-data for it in a mongo db instance. However for Power BI it stores it's meta data in the PBIX file. If you change the file extension from pbix to zip, you can navigate in inspect the contents.
When the report is deployed to the Power BI Service, it uses a number of components to store the file and meta data, blob storage and a small SQL instance in the background. You cannot access these items or the data in them.
For on premise versions of Power BI, Power BI Report Server (available in Premium only, or some enterprise licensing), this requires a SQL Server database to be used. This acts as a meta data store for the Power BI Front end and, also stores the files etc for the reports loaded to it. You can access this data meta store. More details on the setup here.
I don't think there is a path to migrate data from the mongo db to the sql, or the service, or the files, it will be a full recreate of the objects from one reporting technology to the other.