使用Jlink（Java 9）进行1级和2级压缩的算法是什么？

Question

文档（https://docs.oracle.com/javase/9/tools/jlink.htm#JSWOR-GUID-CECAC52B-CFEE-46CB-8166-F17A8E9280E9）说：

Level 1 -> Constant String sharing
Level 2 -> zip 

我的问题是：

1). Which strings get shared ?
2). Who share these strings ?
3). Compression in Level 2 does not create a zip file. Then how it is a zip compression ?