How can I insert a word document into another word document or join two Word documents together?

Microsoft Word 2016

1. Open the first document
2. Place the cursor where you want the second document to be inserted
3. From the Insert tab,  Text group, click on the down arrow next to Object and choose Text from file.
4. Select the file to be inserted
5. Click on Insert

Note:  If you also want to insert the header and footer, be sure to insert the text from the file in a new section so that the header and footer is applied to only those pages

Use the Save As... command to save the combined document under a different name if you need to retain your original document.