Hey timmy159,
Those are some great questions! Follow these steps to take a screenshot with your iPhone 12:
1. Press the side button and the volume up button at the same time.

2. Quickly release both buttons.
3. After you take a screenshot, a thumbnail temporarily appears in the lower-left corner of your screen. Tap the thumbnail to open it or swipe left to dismiss it.
For all other iPhone models with Touch ID, check out this article: Take a screenshot on your iPhone
To answer your second question about iCloud Photos, check out this article to learn more about how iCloud Photos works: Set up and use iCloud Photos
If the photos are deleted from iCloud, it will still remain on your iPhone but it won't be backed up on iPhone until you enable iCloud Photos again under Settings. If you turn off iCloud Photos, it will save the photos on the Photos app only on the iPhone.
We hope this helps!
Best regards.