Twoultras
If you want to have different apps on different Roku devices, then you need "unlinked" Roku devices. (seperate Roku accounts).
You simply factory reset the device and create a new Roku account (different email address) for that device. https://support.roku.com/article/208757008
When devices are "linked" (same Roku account), then any removal or additional of an app would remove/add that app from all the linked devices. https://support.roku.com/article/360000321787
If you want to share paid subscriptions amongst your unlinked Roku devices, make sure you subscribe directly with the subscription provider, and not through Roku.
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"So there’s no way to treat each device individually as far as Apps if they are all logged into the same account?"
Depends on the app. Is there a specific app or behavior you are trying to achieve on one device but not another? As a general rule, you can/maybe modify some app settings per device but as a general rule ineracting with an app will apply viewing history and other settings on an account basis, not device basis.