How imbed code on multiple web pages such that updates to the code are immediately present on all pages?

Ok, you'll find this on the left menu under Manage > Advanced. 

I don't see Manage in any left menu. Actually, although I thought I saw Website Builder on one of my working pages, now I see only Websites Manager. I click on "Websites Manager" in a left menu to start editting. Once I click on it, I choose "Edit Sites" after which I no longer see a left menu.

Once you click on Edit site for the site in question, what do you see on the left once the builder loads?

First, I'll give the URL that "Edit Sites" takes me to. It's

https://sitebuilder.homestead.com/~site/builder/

What I see on that page are two strips then a rectangle.

On the left in the first strip, it says

"homestead", than to the right, it says "Websites Manager"...

On  the left of the second strip, it says

"View Page", than to the right, there are a number of drop down tabs

The third and last item going down the left side of the page is a rectangle for the page html.

OK, so this is Sitebuilder Plus. You'll need to add the code page by page and it will be under Page Settings > Advanced  Settings