If a Lambda function hasn't been scheduled yet, how does it become the first to execute, and what bridges the functions together?

A Lambda function that isn’t scheduled by a CloudWatch Events rule or EventBridge rule will only run when it is invoked directly (e.g., by another Lambda, API Gateway, or an S3 event). The "first to execute" is determined by the trigger that initiates it. If you want a function to run automatically on a schedule, you create a CloudWatch Events rule (or EventBridge rule) with a cron or rate expression and set the Lambda as the target. The rule acts as the bridge: it watches for the scheduled time and then invokes the Lambda. If you have multiple functions that need to run in sequence, you can chain them by having the first function invoke the second via the SDK or by publishing an event that the second function’s rule listens for. Thus, the bridge is the event source (S3, API Gateway, CloudWatch Events, or EventBridge) that triggers the first function, and the invocation mechanism (direct call or event) that connects subsequent functions.