Channels are sync'd across devices, but not user credentials. Those must be entered on each device. That's a security feature, as your credentials are not stored in the Roku cloud where they could be hacked.
While each device linked to the same user account will have the same channels installed, there are a couple of caveats. First, if a device won't support a channel that works on other devices, then that channel won't appear and cannot be added. The second is that while the channels are sync'd, their location on the home screen is independent on each device.
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