If I turn off iCloud Photo Library will all my pictures from iCloud be erased? And will pictures from my iPhone from then on not be stored in my iCloud?

if you turn off the iCloud photo library on you phone the pictures will not be erased in the iCloud

you can always check what is in you iCloud on the page iCloud - Sign in

if you turn off iCloud photo library the photos will not be saved anymore in the iCloud

iCloud Photo Library - Apple Support

to know what goes in an iCloud backup you have What does iCloud back up? - Apple Support