What is this file “ mobile Downloads.lrdata “ for

It's the storage location of all the photos that are downloaded from the cloud to Lightroom Classic, i.e. the photos you shot with your phone. It shows in the Lightroom Classic folder panel as a separate device; like a separate hard disk. You can change this in the Lightroom Classic preferences, but that will only change the location of new photos. If you want to move your existing phone photos to another location, then you have to do this with drag and drop, just like any other photo you want to move to another location. Don't just delete it, because then you will delete the originals! After you moved the photos and changed the location, then you can delete it. It does not contain edits (they are in the catalog) or previews (they are with all the other previews).