My understanding was you couldn’t even have extensions on iOS, except for ad blocker style things which are explicitly limited by API to prevent blocker from knowing what it was blocking due to tracking concerns.
No, there are extensions for both macOS and iOS. For iOS the user needs to explicitly tap the extension for each page they want it work on, while on macOS it can run on every page once it’s been approved.
I may be wrong about safari on Mac iOS.