They are probably going to make them obvious at first, like with Google search.

But they can also make it more pernicious. For example by having companies pay them so that they can train their AI on their products, with regular updates. Not technically an ad, but the AI will be more aware of their products so that they are more likely to be recommended to the user. In other words, that's paying for the right to advertise to the AI rather than to advertise to consumers directly.

probably, probably and more probably.

Why would they ever make it obvious? It makes no sense. google just had the luck of political inaction, and eventually enshitified it further to the point where you might not know it was an ad.