Questions Regarding nnU-Net

I would like to ask a few questions regarding the nnU-Net framework.

What is nnU-Net, and which preprocessing steps are automatically performed by the framework?

Does nnU-Net automatically apply:

Spatial normalization,

Intensity normalization,

Image resizing/resampling,

Any other preprocessing operations?

What is the difference between U-Net and nnU-Net?

Thank you for your time and assistance.