What brand smart thermometer have you bought?
I have the Nest thermostat.  It works very well to control the temperature of my home.
I have read that the Nest Temperature Sensor that goes with the termostat is handy, it takes temperature readings of different rooms in your house.
I don't have one of those, but have thought about getting some.
What other smart devices do you have?
I have smart plugs, a smart doorbell and a smart vacuum.  
do they all connect through a common system like the plug to an Alexa or the doorbell Nest system to your thermostat?
The plugs and vacuum both connect to Alexa.  The doorbell is ring brand.  I haven't connected the doorbell or thermostat to Alexa, though I bet I could.
I didn't know that smart vacuums that connect to the Alexa were available. Is it a manual vacuum or a robot like a Roomba? 
It is a Roomba.  I can have Alexa start it or tell it to go home.