Download JD-GUI to open JAR file and explore Java source code file (.class .java)
Click menu "File → Open File..." or just drag-and-drop the JAR file in the JD-GUI window runtime-kotlin-iosarm64__kgp_1.9.20-0.5.1-preview.1.9.20-RC2.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.