As I agree about the fact that the engine is WebKit for any browser, that was not the point you made, I did not spread misinformation. My claim is still true, you can choose your default browser, however you cannot choose a browser with an engine other than WebKit
FYI, WebKit is open source.
I’ve just looked up to this and found safari web extension so there is definitely plugin for safari on iOS and MacOS You can even convert manifest v2 extension to safari extension so I guess the scope of extensions should be similar.
However it look like a pain in the ass to make as it look like each extension need a dedicated app installed. You can look up tampermonkey website and see that plugins do exist.
Moreover you can do quite cool thing with the shortcut app like enable PIP (and keep video playing when the phone is locked) on YouTube without YouTube premium by executing JavaScript directly on YT
Extension for safari seem to exist since 2010 with safari 5