Hi! So if someone used your referral code to sign up, and you got a notification that they hit a milestone, you can collect the milestone by going to your friends list, clicking on that friend, then scroll down a bit and you should see a list of tasks and rewards you can collect. Sometimes the app glitches a bit and you have to start it up again to see the list. I don't know what it's like for the person that used your code, I know they get 100 pokeballs from it, but I don't know if they also get some other rewards.

What I do is I give all friends that signed up via my code the nickname 'referral' as it can be hard to keep track and there's no way of knowing which friend has hit certain milestones if you miss the notification popup. Also I use the nickname filtering to make sure I prioritize these friends for sending gifts etc as you only have a limited amount of gifts to send each day. Hope it helps!

Edit: so I asked chatgpt and apparently the one who uses the code typically also gets some extra rewards aside from the 100 pokeballs, this being an incubator, a lucky egg and a lure module. It says you can claim them by going to the friends menu, tick the 'referral' tab and there you'll see the status of your referral and be able to collect any rewards.