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Developing Portable Programs
Bookshelf Analogy
A CPU Instruction Set Architecture is like a bookshelf pattern.
An Operating System is like a pattern of arranging and retrieving books.
So a bookshelf can hold books arranged in different patterns.
The compiler arranges books in a specific pattern within the bookshelf.
A runtime retrieves the requested books from the bookshelf because it knows the pattern in which the books are to be retrieved.
Typically OS-default runtimes only understand the calling conventions of the specific operating system, and even that is built for a specific architecture.
Java and similarly interpreted languages are an exception to this, as they are run atop virtual machines with backends for different bare-metal CPUs and OSes.
Candidates: LLVM, WASM, and the new Cosmopolitan C Library.
Also related: Developing a Web Browser (Layout Renderer, Style Renderer, Script Processer)