Site collections and site hierarchies have been part of the SharePoint experience for quite some time. SharePoint Server and SharePoint Online share much of the same framework, so many of the design guidelines and recommendations are similar. However, SharePoint Online features one new structural paradigm that doesn't really exist fully on-premises: the hub site.
We can divide SharePoint architecture generally into two categories: classic and modern.
Classic SharePoint
The classic architecture is what we're familiar with; it has a curated look and feel and is typically static. Classic sites and site collections are built using a wide range of specialized templates, grouped into three families (Collaboration, Enterprise, and Publishing). The classic architecture is available in both SharePoint Server and SharePoint Online.
Let's look at the following diagram of a classic site architecture:
Content and applications...