Serverless is not just about http API's. There are a whole range of events in AWS that can trigger a lambda function. This article will go through the basics of a a few possibilities. This is by no means exhaustive and meant to be a starting point for further investigation.
This assumes you have a basic understanding of Serverless and can edit and deploy a Serverless application. If not or if you need a refresher head over to the first Serverless project article first.