As of late 2019 it is now the responsibility of the device manufacturer to create and maintain the integration. This is why you now see so many new brands listed in the new SmartThings app.
(Note that, as with everything else, the first rule of home automation applies: “the model number matters.“ just because the brand is listed does not mean that all of its different models will work with the integration. That’s true even of the big brands like ring and Arlo. You have to drill down or ask the manufacturer to find out which are included.)
If the brand/model you want isn’t listed there, then you need to search this forum to see if anybody has designed a custom integration for it.
If it’s a Wi-Fi device, you may be able to get a partial integration either with Ifttt or with Amazon Alexa routines. See the FAQ: