Stop screenshots on my macbook appearing on my ipad
This is probably universal clipboard as described in Use Universal Clipboard to copy and paste between your Apple devices:

The content is automatically added to the clipboard of your other
nearby device. It remains there briefly, or until you replace it by
copying something else on either device.

It also mentioned in last paragraph of Take a screenshot on your Mac:

To copy a screenshot to the Clipboard, press and hold the Control key
while you take the screenshot. You can then paste the screenshot
somewhere else. Or use Universal Clipboard to paste it on another
Apple device.