At the present time, Alexa offers an option which Google home does not. That is, you can create an Alexa routine (not a smartthings routine) which can use a sensor triggering as the “if“ and then almost anything that Alexa can do as the “that,” including turning on another device. So this is a good way to create an integration between smartthings and another device that Alexa can control. 
.
Here’s the community FAQ on that method. Unfortunately, however, as of the time of this writing (March 2020) google has not offered anything similar. They have promised to add more routines functionality sometime this year, but no details on what it will be yet.