When requested for a review, you re notified by mail , slack or simply github notifs. 
You then need to go and leave you review,  to do that, you click on the notification

it ll open the merge request, you click on add your review

on the + button, it s to leave a comment on the changes
when done, you can either add it as a simple comment, or just the start of the review you can see the difference when using either, the first one is a simple com, the second is start a review, which leads to this "pending" status , Then you click on the top right icon "finish your review"

here you specify if you want to request those changes, or approve, or just leave a whole comment

this is how it looks like when changes are requested. and by branch rules, he ll need to resolve it, and re-request for your review

once done and you approve, this is how it ll look, and the merge can now happen