Instructions for spooling up an instance

I understand you create a flavor and an image (which can be customized with the properties from https://docs.openstack.org/glance/latest/admin/useful-image-properties.html) which can then be fed to openstack server create thingie. But, where is the script (think something like kickstart to see what I have in mind; if I am off, I am off) that defines how the hard drive will be partitioned and which interfaces (if more than one) will be turned on and which will be left untouched. You know, housekeeping things that are done before you feed the "--user-data" stuff. Where is that defined? Can I customize it?