These files show how to reference prims directly.
reference_prim_in_other_file.usda, usdrecord 22.08
reference_prim_in_other_file.usda contains three prim references in other files:
Cube_with_reference references an existing meshCube_invalid_reference references a non existing mesh in a valid fileCube_invalid_file_reference references an non existing mesh in an invalid file
reference_prim_in_same_file.usda, usdrecord 22.08
reference_prim_in_same_file.usda contains two prim references within the same file:
Cube_with_reference references an existing meshCube_with_invalid_reference references an non existing mesh