Do companies have "contracts" for ads on the internet these days? I thought most ads on social media, Twitter included, were self service with an automated bidd

Do companies have "contracts" for ads on the internet these days? I thought most ads on social media, Twitter included, were self service with an automated bidding system that allowed each advertiser to tweak how much they're willing to pay per ad view in real-time. You can essentially just login to your account and hit the big "OFF" button whenever you want.