When the user opens the channel action sheet or topic action sheet (e.g. by long-pressing on the channel or topic name somewhere), the resulting bottom sheet should show at the top the name of the channel, or the channel and topic, respectively.
References
Figma design
Channel action sheet:
https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=6327-96692&m=dev

Topic action sheet:
(I didn't find a Figma design for this in a quick look.)