No start up disk and cannot install ios, cannot find missing storage

I have an Imac late 2015 model. I upgraded and planned to reset it and delete bootcamp and the window partition off the mac and have a single storage disk of 1TB.

Firstly when I try to boot up, it cannot find the startup disk and has a ? like the first image here:

If your Mac starts up to a question mark – Apple Support (AU)

I reset go to Command+R Disk utility and connect to hotspot and my Disk utility looks like this (cannot see where the other 250GB is...which should have been my window partition through bootcamp) :

When I try to download ios, this is what I get - no start up disk to install on:

I've been trying to work this out for days and even put it in the too hard basket. I can't repair anything... When I run first aid, it does need repairs, but I cant seem to repair...

I'm at a loss of what to do - any ideas? I would just like to reset to factory settings - the entire 1TB to be unpartitioned.

Thanks