Templates - .dot
.dotm files are templates - macro-enabled
.dotx files are templates and cannot hold macros
.dot files are legacy template files that can hold macros
All three of these file types can hold AutoText. The first two can hold other kinds of Building Blocks as well.

Documents - .doc
.docm files are documents - macro-enabled
.docx files are documents - not able to hold macros
.doc files are legacy documents that can hold macros
None of these file types can hold AutoText or Building Blocks.

You cannot change a document into a template by simply changing the filename extension in Windows. You can change a document into a template by using the SaveAs command within Word under the File menu.

All files of the document type must be attached to a template - normal.dotm or normal.dot by default. AutoText and other Building Blocks can be stored in the attached template.

The terminology can be tricky but is important.

Template is a term of art in Word. It does not mean a sample document or form, although it can be that. It means a file that Word has designated as a template. The default action for a template that you double-click from Windows is to create a new Word document based on the template. The default action in Windows when you double-click on a document is to open the document for editing. Templates in Microsoft Word - read this!

Autotext and other Building Blocks are things that MS has chosen to put on a QuickParts menu along with Fields and Document Properties (which can be in documents). Automated Boilerplate Using Microsoft Word , Using Fields in Microsoft Word.