Best practices for cross-domain tracking

I'm curious about the different strategies SaaS founders are using for cross-domain tracking, especially when dealing with a separate signup subdomain. How do you effectively track the origin of your signups (e.g., Google Ads, organic, direct, affiliate/partner) and their subscription conversions? I'm facing challenges in distinguishing detailed source attribution beyond basic traffic and signup counts. Keen to hear your experiences, challenges, and any unique solutions you've come across or developed.