I currently have an index.html which doesn't have much in it and want to access the app.html which is in a subfolder from index.html as shown here.
My other html file is in app. In addition how do I reference that page in say a button on my index file?
Your app.html is located here: app/app.html
Try linking to it like so:
<a href="app/app.html">App</a>