Q: is icloud photo library cross platform?

Yes. If you want to do so on your Windows, you'll have to install iTunes on it and then install iCloud. Just make sure everything is enabled so that it backs up your photos.

You can access iCloud Photo Library using the web interface from the Photos.app at https://www.icloud.com.

As long as you are using a recommended web browser, yes, see this page:                Use iCloud.com      

and for the recommended Browsers:  https://support.apple.com/en-us/HT204230

Yes as clearly explained in the links Léonie posted

LN

There is more than one way to skin a cat!

LN

thank you.  The itunes post

in the thread confused me.

It depends on the cloud service you are using.  The web interface will give you access to the iCloud Photo Library, and also to iCloud Drive. You can use iTunes and the iCloud panel on a PC, if you want to access Shared Albums (Photo Streams) from a PC (see: iCloud: iCloud Photo Sharing FAQ). Shared albums are stored in a different part of iCloud from the iCloud Photo Library.  iCloud has many different services, so it is a bit cloudy.

Good Luck!