> Roughly twice per second, a Roku TV captures video “snapshots” in 4K resolution. These snapshots are scanned through a database of content and ads, which allows the exposure to be matched to what is airing. For example, if a streamer is watching an NFL football game and sees an ad for a hard seltzer, Roku’s ACR will know that the ad has appeared on the TV being watched at that time. In this way, the content on screen is automatically recognized, as the technology’s name indicates. The data then is paired with user profile data to link the account watching with the content they’re watching. (https://advertising.roku.com/learn/resources/acr-the-future-...)

Note that this isn't documentation of their data collection practices, this is ad copy providing one example intended to convince advertisers to pay them. They mention TV but my guess is that this applies to every type of roku device, exploiting your data is how they make their money after all, although a stick is probably not capturing the screenshots in 4K. I would also assume that this applies to all content viewed on the device regardless of the app being used and including personal content cast to the device.