JDK 14 doesn't have rt.jar with it. Where are 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.

Allow me to quote the migration guide:

Class and resource files previously stored in lib/rt.jar, lib/tools.jar, lib/dt.jar and various other internal JAR files are stored in a more efficient format in implementation-specific files in the lib directory.