What does a nuget package actually consist of,apart from the compiled libraries?

Take a look here.基本上它是一个.nupkg扩展名的文件，只不过是一个.zip文件，它包含前面链接和一些xml元数据中解释的结构。

Is it possible to download these packages without using the package manager and use them elsewhere?

当然，只需下载.nupkg文件。