Native libraries are typically packaged inside a jar so that everything works over the existing build and dependency management systems.

For example, each these jars named "native-$os-$arch.jar" contain a .dll/.so/.dylib: https://repo1.maven.org/maven2/com/aayushatharva/brotli4j/

JNA will extract the appropriate native library (using os.name and os.arch system properties), save the library to a temp file, then load it.