For example, can I use the C backend to compile zig to C, and then use the system compiler as I would normally do with a meson cross file or CMake toolchain file?
It's possible, but:
1. The C backend isn't 100% there yet. You won't be able to use all features and might run into bugs.
2. The generated code won't be very readable, it's arguably not too different from just using Zig-compiled object files directly in terms of "opaqueness" and legibility.
If neither of these are a big problem for you (both points are likely to improve with time), then yes, you could do that.
The idea would be that if C files are reproducible across multiple environments (esp. 32 vs 64 bit) the end user would not need a zig toolchain.
In particular, I explicitly don't want to rewrite 15000 lines of build system code, so anything that uses build.zig is a nonstarter.
zig build-obj file.zig
Is it possible to write zig modules which would be liked to the bigger project otherwise written in C, C++ & c?
And moreover, to produce C "blobs" from zig sources which would be then part of the bigger project written in other languages?
I'd also like to know the answer to both!