Hang on, what I'm being told surely can't be correct.
Well it is - the fact that you do not understand does not mean it is incorrect -- the reason you came here is to get help is because you do not understand - now you are arguing with two different very experienced users who are trying to help you - you can not resolve things by arguing with people who know more than you do - you either listen and learn or keep doing it your way which is not working
Lets assume that I have my iPhone syncing to iCloud.
Once again there is no such thing as syncing to iCLoud - iCLoud has many services and you have to only reference the one you are using - which according to you is iCLoud Photo Library on your IOS devices and iCLoud.com/Photos from your Mac - so as has been explained to you this will sync your IOS devices to each other and to iCLoud.com and noting will go to or come from your Mac except what you manually upload or download from iCLoud.com
The whole idea would be then to sync iCloud with my iMac, correct!?
If and only if your want all libraries on all devices to be the same - which you state you do not - once again for all devices using ICPL the libraries on those devices will be kept in sync and all identical - for devices not using ICPL nothing will be uploaded or downloaded automatically - you have to do it manually - very clear and simple
I have hundreds if not thousands of photographs and videos on my old windows pc.
If I transfer them to my iMac they will all be uploaded to my iCloud!? The storage needed would be huge!
And again if and only if you are using ICL on your Mac - again for all devices using ICPL every thing is kept in sync - all additions, all deleting and all edits adn the storage requirements are what they are with a Max of 1TB - I use 200MB and have about 20,000 items - other use more and others use less depending on what they have
If it does work as I am being told it kind of renders the system useless.
Of course it works like you are being told - why would you think we would lie to you and it is not at all useless - it is amazingly useful as it keeps everything in sync - I can take photos with any of our IOS devices and upload camera photos to any or our Macs or IOS devices and the photos are synced everywhere - even when I am on vacation all of my vacation photos sync back to my home Mac and are automatically backed u by TimeMachine there - ICPL is unbelievably useful
iCloud can only just be a back up of my phones photo
Again no - if you are using ICPL is it not a backup (there is an iCloud service to backup IOS devices that has nothing to do with ICPL in any way)
I will have to manually load them into the iMac
No you choose to - if you choose to use ICL everything is automatic - if you choose not to use ICPL then nothing is automatic - you do have to do either, yoou make the choice
Or am I really missing something obvious......I really hope so....
Yes you are missing a very obvious thing - that you do not understand what you are doing or how it works and you insist on refusing to listen to people who do understand when they explain it to you and tell you haw to do things. And when they tell you that there is no service on iCLoud to download photos from iCloud .com automatically but not upload any - it is a full automatic sync or a manual upload/download of photos you want to have on iCloud
Everything was explained in my original post - nothing has changed nor is anything any diferent than that post with the Apple links giving details
THere is no service named iCloud Photos
Which iCloud service are you using for this? iCloud Photo Library? iCloud Drive? My Photo Stream?
If you use iCloud Photo Library then everything on devices with ICPL on are synced completely
Again the only way to access iCloud Photo Library without syncing is to use a web browser
You can sync IOS devices using WiFi or a USB cable - that is not using iCLoud in an way
Sync your iPhone, iPad, or iPod touch with iTunes using Wi-Fi - Apple Support
Sync your iPhone, iPad, or iPod touch with iTunes on your computer using USB - Apple Support
You need to listen and learn or go take a class - arguing here is accomplishing nothing for anyone
LN