Approaches to Achieving Message-Market Fit in Unsaturated vs. Saturated Markets

Hi When building outbound campaigns, we usually rely on onboarding forms, case studies, past campaigns, and customer insights to shape our messaging. If we don't have that data, we identify the ICP and test different campaign angles until we find something that resonates. My question is about message-market fit. For unsaturated markets, is message-market fit mostly achieved by understanding the ICP deeply? For saturated markets, how do you approach finding message-market fit? Is it mainly through testing different angles, or do you follow a specific framework or process?