Trying to understand the relationship between master templates and the templates that use it. If I edit the master template and some of the templates that use it are in an active campaign, will it change those templates and everyone who enters that step after that I complete the change will receive the newest version?
Yes, your understanding is correct. If you modify a layout, any changes made to it will apply to the emails that use it from the moment you press “Save Changes”. All the emails sent from that point onwards will include the changes you’ve made, all the ones sent thus far will have the old version.
Extra useful fact: modifying the email action used to trigger drafts in a campaign also causes all drafted email deliveries to update.