The base FHIR specification defines a set of structure definitions that establish the structure of the FHIR resources (such as Patient and Observation) and their elements.
In the FHIR type framework, a structure definition defines a type which can be derived (profiled), either by specialization or constraint, from another type. This establishes a hierarchical relationship between the types (a tree of structure definitions). The top-level structure definitions in the tree are the structure definitions that do not have a base structure definition. (In FHIR R5 and above, the only top-level structure definition is the Base structure definition.)
To visualize the full hierarchy of FHIR data types and resources and make it more accessible, I've created this FHIR Spec Viewer tool that displays the tree of structure definitions defined in the FHIR specification. You can view each structure definition as a TypeScript-like type definition, open element descriptions by clicking on the purple links, and navigate to the referenced structures by following the blue links.