Why are all my photos so low-res in photos library

What do you see if you go to iCloud.com/Photos using a computer browser? You probably need to let the sync complete before attempting to make any changes.

Has there been any change since your post last night?

You can try starting over. Try going to System Preferences/iCloud (Apple ID for Catalina) and stop Photos syncing by unchecking the box. Wait a couple of minutes, then check syncing. Allow time for the sync to complete.

Last resort  - have you tried signing out of iCloud and then sign back in?

As per the article https://support.apple.com/en-in/HT204264

When you turn on iCloud Photos, the time it takes for your photos and videos to upload to iCloud depends on the size of your collection and your Internet speed. If you have a large collection of photos and videos, your upload might take more time than usual. You can see the status and pause the upload for one day.

• On your iPhone, iPad, or iPod touch, go to Settings > [your name] > iCloud > Photos. You can also open the Photos app, go to the Photos tab, and scroll to the bottom of your screen.
• On your Mac, open the Photos app. Select Photos in the sidebar, then click Photos or Moments in the list of tabs in the toolbar. Scroll to the bottom of your photos and click Pause.

It depends on connection speed, what else the computer is doing, and a few other things, but even under the best conditions, the photos sync is very, very slow. People with large Libraries have posted it took 2 weeks or more to complete the sync. If you interrupt the sync, it starts over at the beginning.