Using Custom Engines for Rendering Engines?

When you were looking at that stuff, did did these bespoke engines that they had, were they all? Did they derive off some other base engine. Is it something similar with these guies, like thee unrail engine, ritic under the covers? Why did they start with one of those and then just tailor it to their needs? Ecause that seems to make a lot of sense,. yes, dannyas lot more sense to me.