If you edit the email in the live journey, it will update the journey immediately to use that new content. This is replacing the previous requirement of pausing, republishing and then restarting a trigger each time content was updated (like you still need to do on normal triggered sends).
This was a great feature that JB has to help you save time and remove the confusion around this process. BUT it does not work if you edit in Content Builder and not in the live journey. Any changes to emails outside of the live journey they are in, will not result in the content being automatically updated. It will just change the email in content builder.
Each Journey or Email Send automatically creates a snapshot of the email (with pre-rendering to allow for quicker execution) at schedule/publish and this is not updated unless the send is either cancelled and rebuilt or paused/republished (which is automatically done in JB when you edit in there).