Its been a few years since I looked. But yeah.. basically all the consumer and general commercial monitoring is centralized to be handled by ADT. Hardware is all from honeywell or a few others. Companies like frontpoint etc handle customer acquisition, support, Billing, and distribution. Most of the “apps” and alarm system configuration is also reskinned ADT.
There are a couple small companies that provide more access to your own hardware. You can use their centralized system to send yourself alerts etc. Having police/fire/medical dispatch gets back them paying ADT to process those alerts. If you want to start in this direction I was happy with suretyhome.com.
Edit: regarding consumer hardware its all either zigbee or z-wave these days. A DIY system (like with surety) is generally all compatible and Id ball park more like $3-400 for hardware. If you use one of the bigger players they use custom firmware & lock down the panel/sensor management to ensure you pay them a nice markup.