So, in the case of a billboard, if you are paying a landlord, that's advertising. If you are paying a newspaper to print a specific article, that's also advertising. This means paid press releases are also not allowed. Product placement would fall under this definition too - if a specific car brand is paying you to feature their cars in a movie, that's advertising.
Notably, it's not advertising if no payment is being made. If you are making a movie and you decide to feature a specific car brand - and you aren't getting any kickbacks for it - that's completely allowed.
It's also not advertising if it's first-party. For example, a sign that's advertising a restaurant is allowed if the actual restaurant itself is underneath that sign. And it's also not advertising if Disney is pushing Disney movies and products at Disney World, because Disney owns the full creative rights to Disney World and they aren't being paid by outsiders to adjust the messaging.
This definition can even be robust to grey areas like "what if a car brand makes a movie featuring their cars?" - well, how is that movie being distributed? Are they paying people to distribute the movie, or is it genuinely a good movie that people are distributing on their own? Paying people to distribute the movie is not allowed, but if the movie is good enough that people are distributing it anyway of their own volition, then it's okay!
Overall, the definition is pretty large, and paid promotion is so deeply ingrained into modern society that it's difficult to imagine exactly how much would change if advertising was banned. But, quite a lot would change! Pretty much the entire playbook for all commercial enterprises for "how to tell the world about your thing" would have to be re-written, and new institutions would have to be developed to replace advertising.
But I think society overall would benefit greatly from the change!
Is it ok that when you're watching broadcast/network TV, they advertise internet or cellular service, because the conglomerate that owns the TV station also owns an ISP and cell carrier?
Is it ok if you're using a popular web search engine, and they advertise their own hosted business productivity suite?
I think no, we should not allow these things. But no money (or consideration, or whatever) has exchanged hands here.
I think that similar exceptions extend to a TV network that's pushing its own products at you while you are watching the station. No consideration has been provided to push the ad, so you are in whatever ecosystem.
How do you know when you've crossed the line into abuse? Well, we have anti-monopoly laws for that. At some point an ecosystem becomes so big it's a monopoly that needs to be broken up, and after it gets broken up it can't self-deal across the broken pieces anymore. So just like we already have good legal infrastructure in place for figuring out when "consideration" has happened, we also have good (well, maybe not good enough lately) legal infrastructure in place for figuring out when a company is too big and too able to self-deal.
So if I pay somebody independent to hand out leaflets, that's advertising. But if I employ somebody in the position of leafleteer, now it doesn't count.