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 daml-lf-2.1-archive-proto-3.0.0-snapshot.20240318.12913.0.v1c415c97.jar file.
Once you open a JAR file, all the java classes in the JAR file will be displayed.