This is ssi.shtml. We used the file name ssi since this means server side include.
You need the shtml but not the ssi name—any name will do.
Here is the text without the include yet. Any text below this paragraph came from the include.
It required an shtml extension on this shtml file but just an html extension on the included file.
Put <!--#include virtual="1bunchastuff.html" --> in the body section since that is where it goes.
The include below is simply the text in the 1bunchastuff.html file. You can load text files this way also.

Here is the text in the 1bunchastuff.html include file. It has no head, body, or html tags. Just the html code.