Mijia :
Mijia is a sub-brand of Xiaomi than you can connect to automation with apk "mi Home".
Xiaomi make partners with other brand, like yeelight (lamp, bulb, ...), Aqara (sensor), hey+ (this band
), recently ikea and other.
That probably why the notification of mihome are lock except the Mijia notification (partnership betwheen hey+ and Xiaomi, it's not a partnership with all other Brand)
You have to know, some brand lock the product outside China.
Gateway xiaomi aqara:
You need the gateway xiaomi aqara for control the product aqara, many sensor (body, temp,...) essencial for a good automation, and that work only with location main china but you can set english language
Btw, Aqara sell a gateway alone compatible homekit but don't work with mihome
BLE gateway :
BLE = bluetooth
When your phone are close to the device on bluetooth it's ok. But when you go out, the gateway Xiaomi don't have bluetooth, so that use some product with wifi + BLE for connect produits who work only in bluetooth, for example the hygrothermograph.
So, for this hey+, if you use mihome on bluetooth with your phone, normally that work (except for automation, cause that need the BLE gateway). And you have only step of the day and last Heart rate, Idk if this product are lock outside China, cause i'm all time in location : main china.