Text boxes and images

Word does not let you add a custom shape to the built-in Shapes gallery. That gallery contains only the shapes supplied by Microsoft.

For a reusable assembly or test-instruction graphic, create the shape once, then select all of its parts and use Shape Format > Group > Group. With the grouped shape selected, go to Insert > Quick Parts > Save Selection to Quick Part Gallery. Save it as an AutoText or Quick Parts building block.

It will not appear under Insert > Shapes, but it can be inserted again from Insert > Quick Parts and remains editable after insertion. If several people need to use the same shapes, save the building blocks in a shared Word template (.dotx) rather than only in the local Normal template.

I join Jamony in recommending creating the desired shape, selecting it, and saving as a Building Block. 

I would recommend saving as a Text Box or AutoText.

If for use in multiple documents or by multiple users, it should be stored in a Global Template (https://www.addbalance.com/usersguide/templates.htm#Global).

See also:

• Where can Building Blocks be stored?
• Startup Folder (https://addbalance.com/word/startup.htm#PageStart)

If you have different shapes that you wish to store, you could also use the Building Blocks Gallery Content Control which can give you a menu of your shapes in your document. That Content Control can, itself, be stored as AutoText allowing you to use the AutoComplete feature to insert into a document.

• AutoComplete
• Building Blocks Gallery Content Control

You want to reuse custom assembly symbols from Word’s Shapes menu rather than rebuild them in every document. Word does not provide a supported way to add user-created entries to the built-in Shapes gallery. In desktop Word, create the object with Insert > Shapes; use Freeform or Scribble if needed, add text or images, then Ctrl-select the components and choose Shape Format > Group > Group. Select the object and choose Insert > Quick Parts > Save Selection to Quick Part Gallery. Give it a name and category, and save it in Normal.dotm or a dedicated template when Word prompts for the storage location. You can then insert it from Insert > Quick Parts in later documents. Quick Parts and AutoText can store text and graphics, making them the supported reusable-content alternative. If several people need the same symbols, distribute a controlled Word template containing those building blocks.