The only jar file I could find inside JAVA-HOME/lib was a file called jrt-fs.jar which is only 105 KB in size.
So where are Runtime (bootstrap) classes then?
The rt.jar
file was removed in Java 9, but if you need to access the classfiles in the runtime, you can do so easily through the JRT file system. See, for example, https://stackoverflow.com/a/53897006/3553087.