Lots of "modern" languages don't interop terribly well with other languages, because they need a runtime environment to be executed.So, if you want to call a Python function from Java, you need to start a Python runtime and somehow pass the arguments and the result back and forth (e.g. via CLI or network communication).

C, C++, Rust and a few other languages don't need a runtime environment, because they get compiled down to machine code directly.As such, you can call functions written in them directly, from virtually any programming language. You just need to agree how the data is laid out in memory. Well, and the general agreement for that memory layout is the C ABI. Basically, C has stayed the same for long enough that everyone just uses its native memory layout for interoperability.

And yeah, the Rust designers weren't dumb, so they made sure that Rust can also use this C ABI pretty seamlessly. As such, you can call Rust-functions from C and C-functions from Rust, with just a bit of boilerplate in between.This has also been battle-tested quite well already, as Mozilla used this to rewrite larger chunks of Firefox, where you have C++ using its C capabilities to talk to Rust and vice versa.