Why does my high quality Photos appear as low resolution on different devices

Are your various devices set for "Download and Keep Originals" or "Optimize ... Storage"? Could you be seeing the storage optimization function at work?

I don't understand what you showed us in the screenshot. Is this your original, or is this the "low resolution" image?

On my 12 pro, a typical 12 megapixel image (same resolution as the one you show) takes 3.4 megabytes. But your image shows the file is only 0.86 megabytes. This seems to be due to a very lossy compression. Are all of the pictures you take like this when you take them? Check the info! When does this change happen? It can happen in email transfers, for instance.

You said "Viewing photos and sharing." I don't think that we understand what you mean. How did you share? Are you using a Shared Album?

So, is this information about the file correct for the original image?

In addition to what Yer_Man suggested, it would help if you could attach numbers to what you're seeing. What was the pixel size of the original picture? And the file size? This is given in the Info Window on the iPad:

And the same information for the low resolution image? That would help.

As we can't see your devices, nor read your mind, we need you to give us the information that will help you. Start with a full description of the issue, on what devices running which operating systems. Then describe how you get the different photos to the other devices, and add in anything you think might be relevant.

Please also answer Yer_Man's question: "describe how you get the different photos to the other devices" so this thread does not start to look like Whac-a-Mole game :-/