Remotes fully interchangeable?

Roku has its "simple remote", which controls via infrared (IR) signals.   All roku devices with IR receivers respond to the same set of IR commands so for those Rokus the simple remotes are interchangeable.  Universal IR remotes that have built in commands for Roku devices are interchangeable for IR equipped devices as well.

Roku tvs come with IR remotes, but they use a different set of IR commands than Roku devices do.   IR remotes are not interchangeable between Roku devices and Roku tvs.

Roku also has Voice Remotes that control by wifi.  Roku Streaming Stick products can only be controlled by wifi, as they are normally used behind the tv where IR cannot reach, so they don't have IR receptors.   Roku TVs and all but some fairly old Roku devices are compatible with these as well.

You might want to list the model numbers (found under Settings->System->About) for each of your Roku devices and someone here is sure to tell you what's compatible.

There are some very old Roku devices for which Roku no longer supplies a remote (like the 3500), but anything in the last few years should have a remote available.

In general, any device that connects via an HDMI cable (and doesn't simply plug right into the TV) can work with any Roku remote.  Any recent stick can work with either of the voice remotes (the 3500 being one that does not as well as the old MHL sticks), but not the simple remote.  Any Roku TV can work with either of the voice remotes, but not the simple remote.