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Run time environments and compilers are essential parts of complete operating environments.
For proprietary operating systems, a single company tends to control the development of it's core components. (Microsoft, Apple, etc..)
For free (as in freedom) operating systems (like GNU), it's typically a community driven effort by the hands of thousands over time. Components are forked and modified by many, and some of those changes are pulled upstream to the project's official repositories. Here's a visualization of commits made to the GNU Compiler Collection since it's development began in 1989: https://www.youtube.com/watch?v=ZEAlhVOZ8qQ
The run time is governed entirely by the operating system. The compiler is a tool that will convert a programming language into machine code.
The operating system is responsible for allowing programs to run on the processor(s), system security, file systems and more. Most of the things you need to run a program.