For this you have to download eg from a series ne episode to rauszuschneiden few scenes, but if I download something at Netflix then it is stored on the site itself and does not go into my gallery or? Is there another way to download yourself?
You can easily record the scene with Screen Record and save it in your gallery.
You need the permission of the copyright owner anyway, so you can ask for permission to provide the appropriate material by requesting this permission
For you privately you can do that, but uploading / publishing violates copyright without license / consent.