So not really 100% domestic since it runs on ARM and is built using C, C++, Rust and JavaScript but they are close. I prefer something more open like Linux or more integrated like macOS, but fun for them.
C and C++ are ISO standards, with multiple implementations.
JavaScript is an ECMA/ISO standard, with multiple implementations.
Rust is sponsored by an international foundation.
Additionally, Harmony OS is its own thing, it has long stop being an Android fork, with ArkTS and Cangjie, as the main way to develop applications for the platform.
The reality is that computer technology is global. It does not belong to any single country.
If you analyze any piece of hardware or software, it aggregates various elements that have their origins in many countries.
What matters is whether you can design and build a complete system, not from where you might have learned each of the great number of things required for this.