Camera all set, waiting for this new message to come in so the audience can read it. The shot wasn't quite in focus, or there's a funny reflection on the screen, or we just don't quite like how it was framed.
OK, let's go for take two… now, how do we get rid of that message & send it again, just when we need it on camera?
Answer… use a mock app which quite possibly is just running video & not a messaging service at all.
Watch how quickly people are capable of replying to such as this too, whilst apparently having only half an eye on what they're doing - never a typo to be seen.
It is unimportant, other than for product placement, copyright or advertising concerns, whether it is a copy of a real app or not. Watch any show on Apple TV & spot the Android or Windows PC… never going to happen unless it's a 'baddie'.
Here's a practical guide to how you can even do this yourself to turn an apparent text conversation into a movie you can play back on-screen - https://www.kapwing.com/resources/how-to-make-a-fake-texting-story-video/