There are multiple ways to link a Folder or Content ( Part,  Document, or CAD Document) to an existing Folder, with the prerequisite that you have the necessary rights to edit this Folder:

1. For the Part & CAD Documents, when these objects are created using one of the CAD connectors. The first Save action allows you to select a Folder and to link either the Part, or the CAD Document (or both) to this Folder.
   Refer to the dedicated documentation of each connector for more info. Example for CATIA PWB.
    
2. When importing a new object from a batch import.
   Refer to the dedicated documentation for more details.
   
   Also, at any time, you can create a link using one of the following method from the PLM:
    
3. From the Profile Card of the folder:
   Edit the Profile Card and expand the Relationship grid. 
   Go to the appropriated tab, depending the nature of the object you want to attach ("Folders" or "Content").
   
   Then click on the "Add Folders" / "Add Controlled Items" button 
   And search for the object you want to attach in the Select Items window. Then click OK to come back to the Profile Card.
   Click Save / Done on the top of the Profile Card so that the changes are saved.
    
4. From the tree navigator:
   From any Folder tree view, you can select a Folder in the tree and from the contextual menu, click on "Pick Item":
   
   Then select the type of Object you want to attach and search for it in the search window.
   
   
   Click OK to come back to the Profile Card.
   Note: you do not have to Edit and Save the Folder using this method, nevertheless, you must have the Edit rights to perform the action.