I recently purchased one of these smartwatches for around $35 including postage from trademe. I had seen the same ones advertised on FB and checked on a youtube video to see an unboxing.
I purchased one for a colleague at work too. Basically It has all the usual functions for a sports band/fitbit but also included BP monitoring and Spo2.
Ill tell you what it is made of plastic for sure but it appears to work reasonably well and for $30. the connection app for the android/IOS is an app called Fitpro. I went for a walk this am and it mapped my walk fine.
The only downside is the recordings are not automatic-on the app you press a button-"start measure"
Ill report back on the accuracy of the Bp readings-we have a cuffed one at home too.
I am on medication for high blood pressure hence me looking for this type of Device.
I have been using the Huawei G2 watch that came with my P30 pro.
you also have notifications from facebook and a few other apps.