Your channel apps are attached to your Roku ACCOUNT. All Rokus linked to the same account (i.e. their Settings > System > About shows the same email address) will get all channels from that account that they are capable of running (some older Roku models may not be able to download and run some of the newer channels).
Any subscriptions you have THROUGH ROKU are valid for all Rokus linked to the Roku account through which they were purchased, so you only pay for one subscription. If you have Rokus linked to different accounts, each account would have to have its own paid subscription for channels subscribed to through Roku.
Any subscriptions you have made directly with the channel services' own websites will be available on ALL Rokus, regardless of what Roku account they are linked to - you just log into these channel apps with your credentials for that channel service.
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Re: Does my roku account connect to all my roku devices in my house or are they paid separately?
To add to what @makaiguy said, there is no payment to activate or use a Roku device. Watch out for scams. Individual channels/apps may require a purchase or subscription, but there's plenty of free, mostly ad-supported, content available.
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