How exactly is libjsc.so bundled with apk?

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Question

We are working on making a reproducible/verifiable builds for our RN app.
Independent security researchers compile app locally and compare with published on googleplay/website build.
We had success making identical js bundle, but the problem is that files libjsc.so differ a lot.
Difference is around several megabytes per each architecture.

So, where do these files come from during ./gradlew assembleRelease ? Are they pulled from repository? Are they built locally, and if yes, what tools are involved? Java,
android build tools, cmake..?

PS. Our production builds are built on appcenter.ms and they don't expose exact software versions on their build containers. I raised a support ticket there, waiting for a response.