how to unzip nested zip file

You can unzip it once, loop through all the files and if any extracted file extension is *.zip then you can unzip it again. Since it sounds like you know how to unzip once, post that it's just logical coding to have nested unzip.
 
Still, in case needed, refer:
How to Zip and Unzip entire folders with nested files/folders.. example C# code[^]
Unzip Nested Zip Files While Streaming[^]