If you'd like to build Open Mobile Maps yourself, make sure you have all submodules initialized and updated. To do this, use
git submodule init
git submodule update
The bridging interface between Kotlin and C++ are defined in the djinni files under djinni. After modifying those files, the new bridging code can be generated by running
make clean djinni
in the folder djinni. This generates the Kotlin bindings, the C++ header files as well as all the Objective C glue code.
The Package.swift file can be opened in Xcode and build directly from there.
Unfortunately, Swift package is only supported starting with iOS 11. If you need iOS 10 support you have to compile the library as a framework yourself.