Back when Cursor was new (before literally everything was AI hype) they explicitly called out that they wanted to do more in-depth integration with the editor than was possible with just the extension APIs.
Presumably that hasn't changed much. If you want to do any large-scale edits of the UI you need to spin up a fork.
I don't know, does Cursor offer anything substantial beyond an extension like kilocode? (I've only used vanilla VSCodium branch with various extensions but they all seem to integrate everything from tab-completion to complete UI agentic take-over very well)
Presumably that hasn't changed much. If you want to do any large-scale edits of the UI you need to spin up a fork.