Ask HN: How do data enrichment tools (Apollo, ZoomInfo) acquire their data?

Hello, I've been involved in the development of B2B SaaS solutions, and there are a few providers, such as ZoomInfo, Apollo, and Clearbit, that greatly assist the sales team in gaining a deeper understanding of customers. However, I'm curious about how these businesses obtain such precise data considering the strict regulations like GDPR. Has anyone attempted to create similar companies that offer Data as a Service?