Yeah bethesdolt is bethesdolt, but the underlying engine is ofc the same, which is why awesomely reactive shenanigans like Sim Settlements can happen.
Yup, but it's still a walled garden that is quite locked up.
Bethbryo: walled garden
Source 2: walled garden by merit of Valve stuffing both thumbs up their collective asses to indefinitely twiddle about any real game development besides chasing fads
Unreal: licensing fees put more money into nuEA starting to become as bad of a rent-taker as Valve
Unity: can be modded but several developers consider it a mistake to continue using it, notably ex-Origin/LGS. It's the reason why Shroud of the Avatar could be exploited if anyone bothered to - but it does a convincing job of being the scapegoat for anything breaking.