The core four
Every AWS service answers one of four questions. Services that answer the same question are alternatives: learn the question, and each new service is just another answer to it.
1ComputeWhere does the code run?EC2 · ECS + Fargate · Lambda · SQS2NetworkingHow does a request get there, and what can reach what?VPC · API Gateway3StorageWhere does the data live?RDS · DynamoDB · S34SecurityWho can do what, and what did they do?Organizations · Control Tower · IAM Identity Center · IAM · CloudTrail · CloudWatch Logs · Config
Every request touches all four. Watch the label on each step of a journey, then swap one service and see what else has to change.