Outlook, and possibly other email clients too, include a “messages in this conversation” feature. If you have a lengthy chain of correspondence with people ping-ponging emails to each other, you can open any one of them, and a click on this button will give you all the emails from the to and fro, even if they’re spread all over your inbox or have been moved to other folders.
Does this feature work by simply looking for text that is repeated across the messages as a result of “reply all”? Or do email clients, when they send a message, embed some kind of identifier in it which is repeated in other messages that are sent in response? The latter would be more efficient and reliable, I guess.
Yes, the latter. This info is contained in the (usually hidden from you) message header portion. Most specifically, in the Message-ID, In-Reply-To and References fields
Of course that depends on everyone’s email client and service playing nice together and doing all the “musts” correctly and nearly all of the "should"s consistently. But in a work environment you and most of your co-workers are probably a tech monoculture so that’s pretty much a given they play nice together. Your outside contacts not so much.