Chrome for Android can't run extensions, but Microsoft Edge for Android can, and Web Highlights runs in it. Add it once from Edge's extension list, and you can highlight every web page you read on your phone.
Note: Edge may tell you that the extension was not tested on mobile devices. You can still add it. Web Highlights is built for computers first; highlighting, notes and the notebook are adapted for touch screens, other screens look like they do on a laptop.
Tip: Tap an existing highlight to change its color, add tags or a note, copy it, turn it into a flashcard, or delete it.

Tap the Web Highlights icon in Edge. The notebook for the current page slides up from the bottom and lists the page's highlights. At the bottom of the sheet, Visit the Web Highlights dashboard opens your whole library in a new tab.
Sync: sign in with the same account on your phone and your computer to see your highlights on both. Syncing between devices is free for a 7-day trial, then part of the paid plans.

More on which browsers support extensions: How to install Chrome extensions on Android.
Yes. Open the web app at app.web-highlights.com in any browser and add it to your home screen. Then share a page from any app to Web Highlights to save it, open it in the reader, and highlight it there.
Not in Google Play yet. The web app at app.web-highlights.com installs to your home screen, and the Edge extension covers highlighting while you browse.
Yes, when you are signed in with the same account on both. Sync is free for a 7-day trial, then part of the paid plans.
No. AI summaries, chat and flashcard generation run on-device in desktop Chrome and Edge, and phones do not ship that on-device model.
On your phone, open Edge and search for Web Highlights under Menu → Extensions. This button opens the Edge Add-ons listing.
View on Edge Add-onsUsing an iPhone or iPad? Read how to use Web Highlights on your iPhone.