You need the hub to connect non-Samsung smart home devices ... Focus on purchasing only compatible brands and products, and you can control every device in your home using the SmartThings app.
Bixby is Samsung's AI voice assistant, and Smart Things is the device hub that provides the interconnectivity between all the appliances. These are devices such as the SmartThings Hub, Motion ...
I’ve written in the past about how one of the disadvantages of Smart Home devices is that they aren’t really inter-operable, and you have to commit to a single ecosystem — Amazon, Google, Apple, etc., ...