How to load, in QGIS, raster datasets originally mosaicked in ArcGIS?

I have used ArcGIS for years in an academic context but have very recently started using QGIS in earnest as my ArcGIS license is expired and I want to experience a new set of soul rending frustrations. I have a large catalog of DEM rasters from the US National Elevation Dataset (in ArcGRID and/or Gridfloat format) that have been mosaicked with ArcGIS. I do not know how to give more data on them, as I cannot access them in ArcCatalog and they do not show up in QGIS’ Browser (at least as files I’m used to seeing or whose structure I can understand).

When I "Add Raster Data" in QGIS and open the file browse (Windows) dialog (with the "..." button), navigate to my geodatabase, I am given the option to choose from a number of file types, none of which seem to be the actual raster/mosaic I’m looking for (like I would see in ArcCatalog). When I click the file type dropdown, I can narrow down the selection, but none of the individual files in the geodatabase folder are of any of those types.

I have ATX(.atx) FREELIST(.freelist) GDBINDEXES(.gdbindex) GDBTABLE(.gdbtable) GDBTABLX(.gdbtablx) LOCK(.sr.lock) SPX(.spx) SUBFILE(.subfile) SUBFILX(.subfilx), but none of these seem to be the actual data I am looking for. Adding any of these filetypes throws an INVALID LAYER error (Invalid Layer: GDAL provider Cannot open GDAL dataset G:[filepath] not recognized as a supported file format. Raster layer Provider is not valid...)

How do I load a raster of this type into QGIS?

THIS LINK seems to address the same question, but I don’t seem to have any .adf files either (at least not with my mosaicked datasets.

I am using QGIS 3.6.2-Noosa on Windows 10