I think this happened when I realized I hated Lightroom CC and wanted to move back to Lightroom Classic. I didn't research it enough, and just started "'migrating" the photos over, or so I thought. I didn't realize Classic would store my files on my hard drive, and I did not come prepared for it. If it does that, I'll just stick with CC and plan a day for downloading my images from the cloud to a hard drive, and then starting fresh using Classic.
BUT FOR NOW, 780GB of space is sitting on my hard drive and messing up everything, of course! If I delete this file, will it delete my files from the cloud as well? Meaning I won't be able to access them on Lightroom CC?
Mobile Downloads.lrdata is the place where Lightroom Classic stores files it downloads from the cloud, and if you sync the Lightroom Classic catalog then LrC downloads everything. If you delete this file, then Lightroom Classic will show the images as 'missing', but that won't mean that they will be deleted from the cloud. Do not delete the images from the catallg however, because that will delete them from the cloud. It may be better to pause syncing, just to make sure.
An alternative is to move the images to a folder on an external drive, in the same way you can move any other image to another folder in Lightroom Classic. You can then also set the destination for new downloaded images to that folder. It's in the LrC preferences.
Yes, the images will stay in the Lightroom cloud ecosystem if you delete the mobile downloads. Just make sure that you do not remove them from the Lightroom Classic catalog while that catalog is still actively synced.