Classification of webcode

for a very long time, I wrote HTML webpages. Every time I coded it as a single page side. Now I want to expand my skills and want to write my first website with more pages, so with an HTML for every sub-website. How is this handled by professionals? For example: I want to have my footer on every page, even at the sub-pages. Should I now copy the footer to every HTML document or is there a another way to have it every side? And how to handle the same for example with the toolbar? Or is the single page website in the future the better way to work with?
~ filip