That depends on how the vendor implemented it. I’m using the Leviton in-wall dimmers and switches and they were added to a Matter enabled Echo Dot 4. I’m still able to use the vendor app to both control the devices and to update their firmware (locally and remotely). I’m also using them in ST via the cloud->cloud integration.
Yeah, that’s what I intended to say, that you have to add the device to the app too, and not only to SmartThings. The app does not communicate with SmartThings to update anything, which was the original question, they are independent control mechanisms.
Then it depends on the brand how you do it, some brands like WiZ don’t allow you to add the lights to the app if you added them to Matter first, they have to be set up in the app first. In others it’s the opposite, you can’t add them to the app unless they’re set in Matter first (but the apps don’t use Matter to communicate with the devices anyway).