Using crates.io is entirely optional, you can download a library's source code and specify the path to it in your cargo config file. (Which is not uncommon in production)

For that matter, using cargo is optional, you can compile rust code using GNU make or shell scripts if you want to. (That's what the Linux kernel does)

I chose to learn C 25 years ago and avoid it now whenever I can. Life's too short to deal with memory unsafety.

Welcome to 1972, young padawan. You have a long journey ahead of you.