Template reference and selection help

Hi there,
I am new to website building and forums, my apologies if this has been covered elsewhere or if its a silly question.

What is the best way to view “frontend” content, then reference it in the WP template library for selection and inclusion my website?

Re default template selection (using the brooklyn theme):
-the default themes are listed and named, which is great
-you can see the “backend” structure of the template before selection, sometimes
-there is no real way to know what they look like and there are a lot of them
-what I want is a way to quickly index pages, rows and columns for insertion
-I have tried looking at the website demo and selecting templates which have the content I want based on the name of the page
-However, the content on the demo website “Demo 03 - About Page” and the content in the WP template library named “Demo 03 - About Page” are different
-this is causing me to have to spend undue amounts of time finding the content I want
-what is the most efficient method?

Any help is greatly appreciated, many thanks!