You create a CloudWatch Logs log group without setting a retention period. What happens to the logs?

The default retention is "never expire". So the logs are kept forever, and your storage bill keeps growing. Set a retention period on every log group, ideally in your IaC (infrastructure as code). If you need cheap long-term storage, export the logs to S3 yourself.

The option "They are archived to S3 Glacier after 90 days" is wrong: CloudWatch Logs never archives to Glacier automatically. "They are deleted after 30 days" is also wrong, because there is no default expiry.