The IR light source doesn't have to be anywhere near the camera (or optics) within reason.
One consideration is matching the light output to the camera.
My experience is with military optics and not trail cams per say. But too much light can wash out optics, to little light and the results are less than optimum.
It also depends on what you are trying to illuminate, some things absorb IR light, while some reflect it. to various degrees.
I have no idea if trail cams have built in light meters, doubt it. More likely the light emitters are roughly matched to the camera.