Where can Flow Log data be stored? (e.g., CloudWatch Logs, S3)

Above both is stored flow log data but some different cloud watch is real time monitoring and S3 is a long term analytics storage using Athena with archival purposes

How can you use Flow Logs for auditing and compliance purposes?

Flow logs is a power tool auditing and compliance in cloud environment .They Provide visibility into network traffic going to and from resources in cloud environment