After you have begun a private meeting in a channel, you can make it public by generating a meeting link and sending it to users outside the channel.
To change a meeting from private to public:
Send the link to anyone you want to invite to join the meeting.
Attendees who were in the meeting when it was private no longer have a connection between the channel and the meeting when it is made public. When they click the Message icon, instead of returning to the channel, they open a message panel on the left.
The content in the right panel is not automatically saved, but you can save it as a text file by clicking the download button at the bottom of the chat: