iBooks

Where is the difference e.g. books downloaded, what shows for redownloading, are they ibooks purchased from Apple or epubs from other sources ?

If what is currently downloaded into the app then you might (if you haven't changed countries since buying them and they are still in the store) be able to redownload your ibooks purchases on the computer(s)/iPad that they aren't on via the Purchased tab in the iBookstore in the app. If they are epubs from other sources then you will need to copy them yourself onto the computers/devices where you want them

If they are PDFs then they won't be in the cloud for redownloading, only ibooks that you've bought from the ibookstore might show for redownloading (as long as you don't change countries and the rights-holder doesn't remove them from the store) - PDFs (and/or epubs) will only be on the devices/computers that you copy them to, so if you want them on your iMac then you will have to copy them to it e.g. by emailing them to yourself, copying them onto a flash drive etc

Yes, only actual ibooks bought from the iTunes iBookstore can show for redownloading on other devices/computers - or can potentially automatically download onto a device/computer if you have automatic downloads turned 'on' on it when you buy the ibook on a different device/computer.

PDFs and/or epubs from other sources you need to sync/copy them yourself where you want them.