s3 trigger for STEP function using Event Bridge

One option is this: https://docs.aws.amazon.com/step-functions/latest/dg/tutorial-cloudwatch-events-s3.html.

You associate an EventBridge rule that listen to s3 events. Then you can execute an step function.

Another option is to use an s3 trigger and react with a lambda function that programmatically starts the step function, but I d recommend to look into the first option, as in that way you leverage aws application capabilities platform instead of gluing flows with lambdas (that you ll have to maintain).