Converging and diverging paths

> Dependencies come from an integrated "SDK" or "Runtime" (these terms are basically synonyms)

They go together in pairs, but are not the same. The developer or packager compiles against a SDK (which is large), and then the end user runs the program against a set of runtime libraries which has been chosen to have its supported libraries and their versions line up with the SDK (but the runtime library stack used by the end user is smaller, because they doesn't need to contain developer-only stuff like compilers, headers and static libraries).

In Flatpak, the SDK is called a SDK, the runtime library stack is called a Platform, and they are both said to be "runtimes". Other ecosystems have different words for essentially the same thing.

Traditional package-based distros do the same, except that instead of having exactly one SDK that is supported at compile-time and exactly one set of runtime libraries that is supported at runtime, any set of packages that meets the requirements specified in the packaging is assumed to be supportable. In distributions that have a distinction between developer and end-user packages, the SDK-like environment has development packages (Debian -dev or Red Hat -devel or similar) but the end-user environment usually does not.