Since conversations in channels are grouped into threads, there are a couple extra shortcuts you can use to navigate in-channel messages.
Use your Tab key to go to the conversation list, then use the Up and Down arrows to move from one conversation to the next.
Press Enter to select a conversation, and then use the Up and Down arrows again to read a message or reply to the conversation thread.
Press Enter to select a message, then use the Tab key to access different actions (like saving or liking a message, or to follow a link).
Wanna bail? Just use your Esc key to return to navigating your conversations.
These same commands work in private or one-on-one chats as well. Just remember that those conversations are simply one big thread, so they’ll look different than your channel conversations.