Questions about Xiaomi Mi Smart

No. Simply the app for control, nothing else.

There is a 2 CM difference in size, 1.5W difference in power consumption (but none in lumen intensity), and of course each has its own home app. Thats it, but basically both are the same product that does the same thing.

I don't think so. It doesn't work with my Alexa.

Nein. >The main difference is that unlike Hue, the Yeelight bulbs don't need a dedicated bridge to work https://www.yeelight.com/en_US/blog/80