Second things first: a car is a big metal box. And a lot of the time it’s a moving big metal box.
Most home automation systems are based on very low power, low traffic messaging protocols like zigbee home automation and Z wave. These can have a lot of trouble getting a signal out of the car. I’m not saying it’s impossible, but it’s not usually a first choice solution.
OK first thing second… It’s really important to note that the standard IOT protocols are:
Zigbee Home Automation
Zigbee Light Link
( read the following if you don’t know the difference between profiles:
)
Zwave
WiFi
Some Bluetooth profiles
Lutron ClearConnect
Lots of proprietary stuff in the 433 and 334 MHz ranges
Of these, zigbee and Z wave have standardized third-party protocols, making it easy for multiple manufacturers to design devices that will all work together.
That brings us to Xiaomi. Xiaomi designed their devices to work with their own hub, not with third-party hubs. They are well engineered, but they are not usually certified to a third-party standard. This helps them keep costs down.
The xiaomi cheap sensors are not certified to the protocols that smartthings uses. Consequently they are what engineers called “idiosyncratic.” Meaning they may work together, they may not, they may do you unexpected things like not work with a particular repeater… People still use them just because they are so cheap but I certainly wouldn’t base a commercial or mass market product on a combination of Xiaomi and smartthings. I would just use the Xiaomi Gateway if I wanted to use their sensors.
See any of the community threads on the Xiaomi devices for more discussion