Ads within a channel not controlled by Roku (The Roku Channel is really the only one they control) are under the sole control of the channel provider. YouTube for example is owned and operated by Google, so they control the ads seen on their channel. Some people believe they see more ads on Roku devices when using YouTube compared to other platforms. That may be true, but it's still Google that's controlling it, not Roku. Remember, Google owns and sells a competing product (Chromecast), so they may make ads more intrusive on Roku platforms. I don't think it would be legal, but someone would have to make the effort to determine if it's true.
When you mention "subscribing" to a channel, I don't think YouTube is one you can subscribe to through Roku. There are a number of premium services that you can subscribe to through The Roku Channel, but I don't think YT is one of them. Remember that if you subscribe to anything through Roku, you can only access that subscribed content on a Roku device. For example, if you subscribe to Paramount+ or Showtime through Roku, you can only access those services via a Roku device. You won't be able to log into those services from a computer or other streaming device.
Roku does offer to channel developers an ad product, but the ultimate control of the ads is still done by the channel developers, not Roku.
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