DWG files are plans (site and/ or buildings) from a Computer Aided Design (CAD) application.
If you do not have a suitable CAD program on your computer you will need a DWG viewer. One free option is Draftsight from www.3ds.com. This is available for most major platforms.
Most browsers will simply download the file to your computer when you click a dwg file download in Flow360.
The one exception to this behaviour is Internet Explorer on Windows which will try to open the dwg file in its own window resulting in a window full of code! The simple solution to this issue is to use Firefox or another browser instead of Internet Explorer.