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On a few old 35mm cameras I think I recall that depending on the lens I used, the focusing aids (split image rangefinder and microprisms) changed a bit.
I now own a couple of Tni DSLRs (both with kit lenses) and I miss my focusing aids even if I rarely focus manually anymore. But I don't know if the lack of focusing aids is because I own inexpensive bodies or because I own inexpensive lenses.
Are split image rangefinder and microprisms part of the lens or the body in modern DSLR systems?
These focusing aids are part of the focusing screen, which is part of the viewing system within the camera body. Some cameras have user-interchangeable focusing screens, others have factory-interchangeable focusing screens, and some cameras have focusing screens that cannot be changed. See for example Canon's leaflet on focusing screens here:
https://www.learn.usa.canon.com/app/pdfs/quickguides/CDLC_FocusingScreens_QuickGuide.pdf
If the focusing aid was part of the lens, it would be visible in the image.
That's because they are optical elements themselves, which means they change the properties of the light that goes through them in order to work. But you do not want them to be visible in the frame (neither film nor digital).
The path of light goes always trough the lens and then either to the sensor/film or through the viewfinder to your eye, depending on the mirror position.
Anything that you want to see with your eye but not the sensor/film should thus be integrated in the viewfinder, not the lens.
