To list something: C teaches memory management, memory addressing, stacks and heaps, some disassembly, binary interfaces. Basically, you see the layer on top of machine language. Python, Go, etc. are a layer on top of that.
Rust and C++ teach these things as well, but C keeps things minimal so you see the ideas more clearly. A lot of languages are written in C, forming a common way to think about implementation and problems, one you see glimpses of working in almost any other language as well. A lot of "why is it like this" questions have answers beginning "because in C..".
The FFI part is due to the extremely stable and simple ABI. Compiling C produces predictable and known binaries, setting a lowest common denominator among languages (for bettet or for worse). Thus knowing the capabilities and limitations of C is useful even when working with other languages.
A lot of this knowledge can be grasped quickly and you will get reminded of them for the rest of your programming life, so it's not necessary to dive deep into C to get the knowledge. C just provides the shortest and cleanest introduction, and you can quickly move forward without missing a beat really, since a lot of the stuff is so universal.
I'd go with C first. Rust would teach you everything C does and more, but C will lay the foundation in cleaner strokes. This will help you later no matter what language you decide to learn next. C is also in a way the language of the FFI, so you are very likely to need it in the future regardless.
Find the kind of system you'd be interested in working on and start building one yourself. For me it was a game engine + an editor for it, but it could be anything you find the motivation to build.
Start with a book or a few texts/tutorials on the subject and begin building. Along the way you'll find the questions and choices involved. Find the answers from the internet or books. This is when I'd recommend some open source projects (not earlier) to see how they solved the specific problems. If you just go into an open-source project you won't have an understanding of the problem, just the answers, so it won't help you nearly as much.
Rust and C++ teach these things as well, but C keeps things minimal so you see the ideas more clearly. A lot of languages are written in C, forming a common way to think about implementation and problems, one you see glimpses of working in almost any other language as well. A lot of "why is it like this" questions have answers beginning "because in C..".
The FFI part is due to the extremely stable and simple ABI. Compiling C produces predictable and known binaries, setting a lowest common denominator among languages (for bettet or for worse). Thus knowing the capabilities and limitations of C is useful even when working with other languages.
A lot of this knowledge can be grasped quickly and you will get reminded of them for the rest of your programming life, so it's not necessary to dive deep into C to get the knowledge. C just provides the shortest and cleanest introduction, and you can quickly move forward without missing a beat really, since a lot of the stuff is so universal.