This should answer most of your questions:
https://help.paramountplus.com/s/article/PD-What-is-the-SHOWTIME-add-on-for-Paramount-subscribers
You don't get bundles like this through Roku, so if you're already subscribed to Paramount+ or Showtime through Roku you'd want to cancel those and subscribe directly with Paramount+ to avoid having two subscriptions to the same thing. In addition, you would view them through the Paramount+ stand-alone channel/app and not through The Roku Channel.
https://channelstore.roku.com/details/32b338eebb16d1de34870b6b1d674911/paramount-plus
