I worked in that industry 12 years ago, and already back then indeed there was a simple method to link some ID (usually cookie) to the injected content. The st

I worked in that industry 12 years ago, and already back then indeed there was a simple method to link some ID (usually cookie) to the injected content. The startup I worked at used it for contests, out of 100k downloads of a podcast, 1 of them would get a different starting or ending segment to give away something. So it was persistent to your user-login on the podcasting portal (which was a thing back then :-))