In AutoCAD, a "Block Host File" is not a single, formally named file type (like .dwg or .dwt). Instead, it refers to the source drawing file that hosts block definitions. When you insert a block into a drawing, AutoCAD either:
Press CTRL + 2 to open the Design Center. Navigate to your Block Host File on the left panel. Click on the "Blocks" folder under that file, and you will see a visual list of every block inside it. You can simply click and drag them into your active drawing. autocad block host file
If you use BIM 360, Dropbox, or OneDrive: AutoCAD Block Host File: The Ultimate Guide to
Layer Management: Put block geometry on "Layer 0" so they inherit the properties of whatever layer they land on in the destination file. Pro: You get large, customizable thumbnails
Here is a breakdown of the utility and performance of managing block host files in the current AutoCAD environment.