The answer you seek, Maren-San, is here:

http://www.photokaboom.com/photography/learn/printing/resolution/1_which_resolution_print_size_viewing_distance.htm

So called "enlarging" software does NOT add detail, only interpolated pixels and some microcontrast enhancement, particularly on edges. It also often adds some undesirable artifacts.

This question comes up several times a week, yet the answer does not change - its all about viewing distance and human visual acuity, which diminishes with increasing distance. A 40x60 print needs only 40 ppi or so to look pretty sharp, which translates to 1600x2400 px, or just shy of 4 mp. Remember the days of multiplex theaters? The great looking Sony 4K films shown in the premium screening rooms are only 3840 x 2160 px, or 8.3 mp. Most people are unaware that with most images you really don't need super high resolution, nor do you have to resort to interpolation software to raise the pixel count - especially if you are anticipating normal viewing distances.

I think you can go much larger than 12 x 16 with RAW images taken with a D500. Images straight from the camera are 13x20. Even so, use the 'Image Size' function in the current version of PS to enlarge to whatever size you are looking for. PS does a great job with enlarging photos.

Just be sure the "Enable Preserve Details 2.0 Upscale" is checked under "Preferences" and then "Technology Previews."

If you don't use PS, you now have another good reason to get it.

Without seeing actual examples together, elbow to elbow, we're left to speak to general principles.

Your 20.1MP D500 should print amazingly detailed images at 18x12in @ 300ppi that seem just as amazing at 27x18in @ 200ppi. All with no manipulation or resizing the image resolution.

Your 48x60in example may have come from a higher pixel resolution camera, say at 45MP D850 or a 50MP EOS 5Ds or a 60MP Sony a7RIV.

I mentioned seeing the actual images with your and our eyes all together. We all could then stand nose to image together and decide if the 48x60in print really is as detailed at the same viewing distance as the smaller prints? Or, as we stand back, are we seeing modern digital images that are amazingly detailed, but 'stretch' the available pixels over larger print sizes? Lower PPI ratios allow an image to appear to have the same detail when viewed from the distance that allows us all to see the entire image rather than nose to print like you can handle a smaller 11x14in print.