previous channel button on Siri Remote?

I guess you’ll need to explain what you mean by ‘previous channel’. The Apple TV, of course, does not have a TV tuner so if you are talking about an app that provides TV channels, like Sling, YouTube TV, Direct TV, Spectrum App, etc., that would be a function of that app.

As for selecting an app that’s already open you can do that by double clicking the TV Screen button on the remote which will bring up all the currently open apps. You can then select which one you want to switch to.

So again, what do you mean by ‘previous channel’?

The software remote on iPhone (or iPad or iPod Touch) does have Channel Up/Down buttons for (linear/live TV) apps that support that.

Set up the Apple TV Remote on your iPhone, iPad, or iPod touch - Apple Support

Control Apple TV with iOS or iPadOS Control Center - Apple Support

to the best of my knowledge you can't customize it at all

suppose each of us use these devices differently I don't have channels I have a netflix app and a amazon prime app and the appletv+ app

So you’re talking about the Spectrum TV app, right. That’s a function of the app, not the Apple TV. You never have explained to us what your are really talking about. Will you ever?