Discussing best practices for cross-domain tracking

I am very curious about how everyone approaches cross-domain tracking, especially from entry on the landing root domain to the registration subdomains. How do you ensure that you effectively track the origin of your sign-ups, whether they come from Google Ads, organic search results, direct visits, or through partners? And how do you track the conversion of these sign-ups into subscribers?

I personally find it challenging to precisely determine where my sign-ups are coming from, especially when it comes to leads from PPC campaigns. Do you have experience with this? I am very curious about your methods and any solutions you have found or developed.