There is a code object called "C Runtime" (crt0.o)[1].
This was a good name at the time. Its a bit of code that gets executed literally "at runtime". When you run a program, this object gets invoked to setup the stack and call main(). Its just a bit of assembly.
Then, decades later, languages like Java overloaded the term "runtime" to refer to their interpreter / vm. This causes confusion to this day, evidently.
The term "runtime" is overloaded here. The C runtime is not what people usually mean when they say a language has a runtime.
There is a crt.0 "C Runtime" object. This is code that is literally run "at runtime". When you execute a binary, the C runtime is the first thing executed to setup the stack and call main().
This is just a bit of assembly that runs before main(). It shares ~nothing in common with "runtime" languages like Python or Java where the runtime is alive during the whole program's execution.
The C run time does not handle floats or threads or anything like that. Software floats are dealt with by the compiler and threading is implemented as its own library (either in userspace or built into the kernel).
Libraries and crt0.o and the like. Libgcc_s ; but it isn’t a runtime in the sense that some magic is doing GC or scheduling for routines. ABI that describes what the assembly should be doing, it isn’t a thing doing stuff.
