I have an inquiry on how to develop a library which other developers using it to build cross-platform mobile apps???
If all the platforms are using java, stick with java. Don’t include android-specific dependencies in it and you should be okay.
If not all platforms are using java, c++ is probably your best bet. Yes, Multi-OS Engine does jni too (objective-c -> jni).
you can explain more (more details)
You can have a look to most github java libraries which may be compatible with moe. I would reckon to use kotlin instead of java for new libraries. For instance, if you check out this kotlin rest client repo: https://github.com/kittinunf/Fuel
it is both working on ios (moe) and android but also desktop application.
You can get some inspiration from it 