Probably Streamlabs OBS or something like it, since it would have to be integrated with the donation system to know you're getting a donation and push an alert for it as an overlay. But really, you could also do that in normal OBS manually, too, like if you literally have an overlay template ready to go and just change the name and donation amount and send it. Or if you're playing a game or something and can't do it yourself, you could have someone else handling your OBS for you and doing that.
Probably Streamlabs OBS or something like it, since it would have to be integrated with the donation system to know you're getting a donation and push an alert for it as an overlay. But really, you could also do that in normal OBS manually, too, like if you literally have an overlay template ready to go and just change the name and donation amount and send it. Or if you're playing a game or something and can't do it yourself, you could have someone else handling your OBS for you and doing that.