Pas group use both for euro 6 vehicles, eloys is very old and no longer used but a new generation of fuel additives are injected into the fuel tank when the fuel tank is mixed to aid diesel particle filter regeneration, urea/add blue is injected into the exhaust again to further lower/improve emissions, useage is based on driving style and type of driving I.e m/way or urban. It’s mostly water that’s why is cheap in the future it will probably go to a solid block that’s heated to extract the urea.
Most manufacturers if not all will be using urea by now for Euro 6/7 cars, but there is not much more we can do to clean up diesel emissions, diesel injection is at crazy high pressures to atomise the fuel, the EGR valves cause issues, most euro 6 diesels will have a urea filter in the exhaust, then a pre particle filter then a particle filter and use additives injected into the fuel pre burn and into the exhaust post burn.
And use complex engine software to control it all.
Small diesels will be gone in a few years with just big lux stuff left purely due to production costs.