"Normal" ads are usually simply tagged as such.

Feed "ads" are a bit more complex, because it's on the content creation side.

Basically, imagine someone wants a specific political candidate to win an election. Instead of making an ad, they would create a profile of the user watching YouTube (or whatever), and create a content specifically targeted for them. This will probably show up in the feed because the algorithm thinks it might be of interest of him.

That way, you create an ad on the feed without it being exactly an ad.

It gets a bit more complicated, but that's the basics of it.