@timtfj @dancingtreefrog that's above my knowledge. I only think in terms of how I move the parts of my mouth to produce the sound. R is a quick tap of the roof, L is more "soft" (sorry, again, not an academic, I lack the words.), a more gentle close/open movement. I'm not sure you can do the same tap with the English L, maybe the Indian R is the closest.

In Spanish and Italian, RR is worse, it's a trill.