Most of those apps and wearable devices aren't regulated the same way as the equipment your gynecologist uses at the clinic. That's what matters: a professional fetal monitor is calibrated, tested, and backed by clinical studies, while many of these home gadgets don't have that same validation. The problem is that if the app gives you a reassuring result, you might trust something that isn't entirely reliable, and if it alarms you for no reason, you create unnecessary stress.
The ideal thing is to stick with your regular checkups with your gynecologist, who's the one who really knows what to expect at each week of pregnancy and can interpret what she's actually seeing. If you have concerns between appointments, it's better to call the office directly instead of relying on an app. Just because your friends use them doesn't mean they're safe or necessary - a lot of times people use things simply because they're available, not because they're the best option. Your doctor is the one who knows your specific case and knows what monitoring you really need.