I really like Fabians source code reviews. Would love to see engines like Unreal being open-sourced to compare how they work internally. Would be a nice perspec

I really like Fabians source code reviews. Would love to see engines like Unreal being open-sourced to compare how they work internally. Would be a nice perspective. I.e. Unreal was much better in out-door scenes and I think the maps where not heavily preprocessed like in Quake et.al. What was their way of tackling the problem? In case there are any resources that describe the technical details, would be nice If someone could post a link.