Trying to understand setup of My Photo Stream to PC

Hello, I'm a photographer and I use a PC. The "Pictures" folder on my PC is rather rammed with my professional work. I do *not* wish to replicate this all to iCloud.

On the other hand I would like my phone snaps available on my Windows PC.

Over the years I've had my iPhone (6s) set up either to backup the iPhone Camera Roll to Dropbox or to the Microsoft OneDrive. Neither of these is particularly elegant as a solution as they rely on me going in to the app periodically or things don't get backed up properly.

A recent service issue has meant I went back to an iPhone 4s for a few weeks.

I've never really pondered the difference between albums "Recents" and "My Photo Stream" before then.

When I got my 6s back again however all of the images I'd been taking with the 4s were on my 6s in "My Photo Stream".

That has made me want to investigate the iCloud again simply for backing up my phone pictures and having them available on my PC (or for the occasional image I want to send from my PC to the 6s).

When I go into iCloud Photos on icloud.com I see no photos or videos at all (neither the ones taken on the 4s nor anything I take on the 6s). I'm not sure how the 4s images got to the 6s therefore.

When I installed the Windows iCloud app I have "photos" ticked and only "iCloud Photos" ticked (not shared albums). The app complains that my windows (Windows 10) is not new enough to support it fully. I find that odd.

I see a folder in "Pictures" for "iCloud Photos" and "Photos" and "Sharing" within that on my PC. Both of those folders are empty on the PC.

Until one moment ago when I looked I had "Upload to My Photo Stream ticked on, on the 6s, which should upload the last 30 days of new photos. I've ticked instead "iCloud Photos" which should now upload everything - I hope! Certainly the folder on iCloud.com has started to get photos added to it as I've done that change, as has the PC folder.

I'm really seeking to understand sending the photos from my phone to the PC (and possibly in the other direction too).

I didn't use the Apple mechanism in the past because it sent all photos in "Pictures" on my PC into the cloud and that didn't work for me as it instantly blew the limits on my phone.

So question 1) can anyone tell me in words of one syllable how to set up this properly as I've got lost in the online links trying to do so 2) can anyone explain why I get a message that my brand new Win10 machine is not a new enough operating system for iCloud to function fully?!

Thanks very much.