Quite an easy answer. The image resolution of the image you have copied from is much smaller than the one you superimposed to. You need to check both images are of same or similar resolution. Go to menu bar, Image > Image Size and check the resolution of each image.

If you are unsure of the area you wish to superimpose to, copy that area into a new document and then check the image size and make a note of it. Then do the same (in a new document) to the area you wish to copy from and then compare sizes. You may need to resample the new document to match that of the one you wish to superimpose to. By sounds of it you need to upscale your new document's dimensions and or resolution. Then copy-paste to the document you wish to superimpose to. It may take a few efforts to get it right.

You could practice by copy-pasting from the second new document with the image you wish to superimpose with into the first new document with the area you wish to superimpose into. These two documents need to match in size and resolution. Note: There are other approaches. Such as after copying the image you wish to superimpose with and then using the transform tool after pasting until the image area matches. This can sometimes be a little tricky. The transform tools are found in menu bar, Edit > Transform > Scale. I'm sure you have experience with the Transform-Scale tool without me needing to point out how (two main methods.)

Keep in mind that upscaling an image will inadvertently reduce the quality of the image. The amount of degradation will depend upon a few factors including what percentage you upscale by and what the original resolution of the image was in the first place. Upscaling degradation is more obvious when working with very low resolution images.

My answers are based on your skill-set to find the easiest answer for you to work with (with less chance of error.)

Joe, again you have posted in the wrong section of the forum. Next time I will not answer your questions until you start posting in the correct forum section. It's not that complicated. Last time I suggested you bookmark the appropriate section but it seems you did not bother to do this. Over 95% of times you have posted on this forum over many years, it's been in the wrong forum section. You have been a forum member for over 10 years. Age is not a legitimate excuse. Even my 92 year old father knew how to find appropriate sections of forums and websites.