That is the “add on” to a parent switch. There are plenty of compatible switches out there:
Just do some searching on this board or search on the internet for “works with SmartThings”. SmartThings supports both Z-wave and ZigBee devices out of the box and there are some other such as Lutron and Phillips Hue that require additional gateways.
Have you had a chance to look at the device class features FAQ? It might give you a better idea of what you’re looking for.
Also, if you really want to make things simple, just stick with devices on the official compatibility list:
As far as accessory switches that don’t have radios in them, the easiest thing is just to choose a compatible master switch ( all the master switches will have radios) and then choose the accessory switches that the manufacturer says go with that master switch if you have places where you need a three-way set up.
Accessory switches that don’t have a radio should all have some mention of that in their product descriptions, like “not a stand-alone switch, requires the master…“ Or something similar. For example, the product description for this which you are asking about in your first post says
Requires a GE Home Automation Switch. This is NOT a standalone switch. (This product is an add-on only switch, and requires an existing GE Home Automation Switch to function. Without a GE Home Automation Switch, the Add-On Switch WILL NOT OPERATE.)
So that’s telling you that it needs a master switch, it doesn’t work on its own.