You can add CAD data directly as feature layers in ArcGIS Pro. When CAD data is added to a map, the content of the CAD file is accessible as read-only feature layers. Once added to a map, these layers can be reorganized and resymbolized.
The default symbology of each feature layer is intended to mimic the symbology of the source CAD layer. The CAD properties of the entities are used to influence the default symbology of the features in the layers. Typically, the default symbology is derived as combinations of the CAD layer name, CAD color, and CAD line style used to symbolize the features as they were in AutoCAD.
You can symbolize CAD feature layers in the same way that you symbolize feature layers from other content sources in ArcGIS Pro to override the default CAD symbology. You can save changes to the default symbology in the layer of the map. You can also save your symbol choices for use in other maps using an LYRX layer file. LYRX is a file format you can use to save and share feature layer definitions. There is a default CAD LYRX directory repository that ArcGIS Pro searches every time a CAD layer is added to a map or scene. If the feature layer name matches an existing .lyrx file in that directory, it will be used to override the default symbology of that CAD layer.
The .lyrx files used by ArcGIS Pro for Civil 3D symbology are in C:\Program Files\ArcGIS\Pro\Resources\LayerTemplates\CAD\en-US by default
Note:
In releases earlier than ArcGIS Pro 3.5, these files were located in C:\Program Files\ArcGIS\Pro\Resources\ArcToolBox\Templates\Layers.
These files are read automatically and applied for each feature layer. The name of the .lyrx file determines which file is used for the matching feature layer in which the .lyrx file is prepended with the label CAD_ followed by the name of the Civil 3D feature layer name. For example, the CAD_Alignment.lyrx file is used to draw the predefined symbology for the alignments into a map, and the CAD_Alignment_3D.lyrx file is used to use the predefined symbology for the alignments in a 3D scene.
In the same way, if you add or replace any of these files with your own symbology definition .lyrx file, the new symbology definition will be applied the next time a CAD feature layer of that name is added to ArcGIS Pro.
Learn more about using an .lyrx file for CAD layers