iCloud tutorial?

New Apple user here. iPhone for about 6 months, MacBook Pro for about 1 week. Never bothered with iCloud until now.

I decided to try it out and, unsurprisingly, I can't seem to get anything to work.

Can anyone point me to an iCloud tutorial that explains all the basics, like what is iCloud, what's it supposed do, what's the difference between iCloud and iCloud drive, etc?

I was hoping that iCloud Photo Library would be an easy way to automatically push photos from my phone to computer, and that iCloud drive would make it easy to view photos that normally live on my computer, on the iPhone. It appears to be nothing of the sort.

Here's what Ive tried so far. I have about ~1000 photos on my iPhone. So far I've just let those photos live on my phone, since transferring files between the iPhone and my Windows computer was a PITA. I turned on iCloud Photo Library on my iPhone (which incurred deletion of hundreds of other photos I had imported from my previous computer via iTunes. They're stored elsewhere on a drive.). My phone spent about an hour pushing 4-5 GB of data to iCloud. I upgraded my iCloud acct to 50GB storage. When I go into iCloud drive on my iPhone it shows 46 GB available and zero items. Im assuming 46 GB reflects the ~4-5 GB space used for my photos (else it would be 50GB, right?). So then why does it claim zero items? I see the same thing when I look in iCloud drive in Finder on my Macbook (but without the available space report.)

Next, I tried pushing a couple photos from a local drive on the macbook into i cloud drive. Those items did appear in the iCloud drive, both on the Macbook and iPhone. When I tap any of the pictures on in iCloud drive on the iPhone, it starts downloading with an option to cancel. The download appears to complete after what seems like the right amount of time (a few seconds for a 7 MB image file), but afterward I can't find the photo anywhere in the Photos app on my iPhone. Where did it download to? How do I see the actual picture and not just the thumbnail in icloud drive?

Clearly I'm lost. Apple's iCloud drive FAQ was zero help. Is there a basic tutorial that help me make sense of this? Thanks!

Edit: I can say that Notes appears to be working with iCloud correctly (or at least, as I expected and can make productive use of. So that's good...)