With earlier versions of Civil 3D I was able to insert dwg files into other open dwg files and all the layers/settings would come along with the inserted file but Civil 3D 2020 seems to treat the inserted file as a totally separate drawing???
Attachment will create an Xref.
Run Insert command, choose the drawing file you want to insert and check the box for Explode. This will insert the other drawing into your current drawing, with all its layers and settings, but not as a block.
Another option would be to right click on the file in and open file manager window and drag it into the current drawing in model space. You be given the options for an Xref or Insert.
An Xref can be turned into an insertion by selecting it and choosing Bind and then Insert from a right click menu. This will insert it as a block that you can then explode.
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