You don't know the difference between lens resolution, sensor/film resolution and combined system resolution.

All you need to know is that either a lens or the sensor or both can limit the resolution of a system. You should also know that the combined system will be less than either of the two components taken separately.

In other words, no matter how good the lens is, it will never deliver more resolution than the sensor. Likewise, no matter how good the sensor is, it will never deliver more resolution than the lens. The formula I presented is an approximation of that relationship.

What I demonstrated is an approximate way to isolate the two resolutions. You can confirm this by picking any* lens on the DxOMark display and selecting different cameras in the "mounted on" drop-down.

* Any lens except where the Perceptual Megapixel is not too close to the nominal sensor megapixels, which it is for all but a handful of the FX mount type lenses.

They actually do. You just need to understand what they are telling you.

The Perceptual Megapixel score combines the lens resolution with the sensor resolution in approximately the following way:

(1/PM) = (1/LM) + (1/SM) where: PM=Perceptual Megapixel, LM=lens megapixel and SM=sensor MP

For example, a 24MP lens on a 24MP sensor would end up with a 12MP Perceptual Megapixel score because (1/12) = (1/24) + (1/24)

But a "perfect" lens (extremely high LP) the 1/LM approaches 0 and PM approaches SM.

To calculate the lens's MP you can rearrange the terms:

(1/LM) = (1/PM) - (1/SM}

This calculation is only approximate. It cannot account for a difference in the actual resolution of the sensor itself which is affected by the Bayer array and whether there is an AA filter or other factors. And it assumes that you are comparing sensors of the same format (size).

But the biggest fallacy is in assuming that a 36 MP camera with a Bayer array automatically delivers 36 MP. But there are only 18 MP of green sensors and 9 MP each or red and blue sensors. The Bayer array actually degrades the resolution by about half and it has to be recreated by the software. A 24 MP monochrome sensor actually starts with 24 MP that have not been degraded by a Bayer array.

However the formulas explain why a non-perfect lens will always degrade the performance of the sensor.

But getting back to the OP's original question, just about everyone is correct that by using only the better center portion of the image circle will result in a higher resolution. You can confirm this with a superficial understanding of MTF curves. The thing that throws a monkey wrench into the works is the size of the pixels themselves and the lower diffraction limits of a crop sensor compared to a full frame sensor.

Your lenses will work just fine on a crop body. I've got a crop sensor Canon 7D Mark II and almost all my lenses are full frame. Because a full frame lens' image circle is larger than a crop sensor, the outer edges of those lenses will not be used. Since in many lenses, there's more distortion and less sharpness towards the edges, you may find you actually get better results using those lenses on a crop camera. Any limitations you run into will be a result of the SL2's lack of features and controls.

Take a look at the first answer under What do resolution figures mean in lens specifications? for a very thorough explanation.

A lens that places an upper limit on system resolution for full frame will do so even sooner when used on a crop sensor. But it will still be much better than a lens originally designed for that crop sensor.

The reason is that a really good lens might provide 100 lp/mm on a full frame sensor (200 lines/mm or 4800 lines over 24mm) but it will also provide only 100 lp/mm on a crrop sensor (3200 lines).

That's the main reason why a Sigma 50 f/1.4 that produces 35 perceptual MP on a D800E and 23 on a D610 will only provide only 16 on a D7100 (see the DxOMark results).

Pick any of the full frame lenses an look at the DxOMark sharpness scores for a D800E and any crop sensor body and you will see that this is consistent.

Larger photosites do gather more light-signal but what the image processor does with that could be another story. That is why the D850 is not being touted as a low light performer - the pixel size is approaching that of an APSC camera so yes my argument does hold. Keep in mind Nikon used a different chip design to help get around the photosite size limitation but eventually the twain will meet for IQ if they keep bumping up the MP - next stop medium format!

If you were to scale up the D500 to FF
D500 20.9 MP x 2.3 = 48 MP, D850 45.7 MP (a FF sensor has approx. 2.3 x the surface area of APSC)

This is why they keep the single digit Nikons to conservative MP levels - allows for larger photosites = low light, fast performance.