DWG is a file format that stands for “drawing”. A DWG file houses all data from a CAD drawing, including 2D and 3D designs, geometric information and photographs. To organize the drawing, the objects are assigned to layers.
For more information on layers, see https://knowledge.autodesk.com/support/autocad-for-mac/getting-started/caas/CloudHelp/cloudhelp/2020/ENU/AutoCAD-MAC-GettingStarted/files/GUID-3C0294C1-4659-42FF-96E7-A5501CCE7046-htm.html