Render Engine is designed to think in terms of pages.
Render Engine's components build off of one another.
At the base level there is a Page, it is just that, a single entity of HTML and metadata goodness.
Sometimes a lot of pages will have a lot in common or belong to a group. We call these Collections. Collections can have any number of the same, or different attributes. Oh and your collections can even have collections...🤯
All of these pages we build need a home. This is a Site. A site serves as a container for your pages controlling it's structure and how pages are built.
Last but not least is the Engine. And engine is what is doing the work building out your pages from the data and templates that you have provided it. How do you start that engine? Simple.