Beginner Questions - Nav bar, and xlm on site

I edited and maintained a webpage for a free online game several years ago using htm. When a major new part of the game came out I had to go through and change the navigation bar on 2k pages. I’ve heard there is a way to put the Nav bar in it’s own file and call on it like using .css style options. I also created an xml document using open office to list enemies in a way to show the best use of time given various factors. If there is a way to create an intractable version of that It would really increase the sites usefulness. Especially if there is a way to change the data on the xml and automatically change values in the creature page.

Atom is only a couple days old to me and I’ve watched some tutorials and done a few searches but did not find anything specific to help yet. As far as hosting the site I’d used herobo and bravehost site before a couple of those free host services since it was just a fan wiki. I don’t know if freehost services allow this sort of backend (if I have the term right) referencing or scriptwork. Any advice and help would be appreciated even if it’s just pointing me to specific threads or tutorials.
-Thank you in adance