Web Highlights is on the App Store for iPhone and iPad. Share a page from Safari or any other app to save it, then highlight it and add notes right in the app. The browser extension itself cannot run on iOS, so the app is how you highlight on your iPhone.
Tip: select a sentence before you save, and the page is saved with that highlight already on it. Text and PDF files can be shared to Web Highlights the same way.

Safari on iPhone and iPad only runs extensions that come with an app from the App Store, and Chrome, Edge and Firefox for iOS do not support extensions at all. The Web Highlights app does not add a Safari extension: you save pages to it with Share instead, as above. This is also true on iPad, even though iPad Safari shows the desktop version of websites.
More on extensions on iPhone: Can you use browser extensions on an iPhone?
Yes. Open the free Reader Mode tool in Safari, paste a link and tap Open in Reader. The article opens as clean text, and you can highlight it right there, without an account.
Yes. Web Highlights is free on the App Store, for iPhone and iPad. Nothing can be bought inside the app: if you already have a Premium or Ultimate plan, it applies as soon as you sign in.
Yes, when you are signed in with the same account in the app and in the extension on your computer. Sync is free for a 7-day trial, then part of the paid plans.
Yes. The app asks you to sign in with Apple, Google or your e-mail address. To try highlighting without an account, use the Reader Mode tool in Safari.
Using an Android phone? Read how to use Web Highlights on your Android phone.