The biggest footprint most people ignore is the hosting. Having all your PBN sites on the same IP range or same hosting provider is the easiest way Google connects them. Different hosts, different IP classes, not just different accounts on the same server.
Registrar footprint is second - same registrar + same whois privacy provider across all domains is a clear pattern. Mix it up, use different registrars, some with whois exposed some with privacy.
Content is where most people are lazy. Same writing style, same post length, same posting schedule across all sites is a pattern. Each PBN site should look like it belongs to a different kind of person — different tone, different niche depth, different update frequency.
The one thing that kills PBNs fastest - interlinking between them. Even one link between two PBN sites creates a connection. Never do it.
Outbound links matter too. Real sites link to external sources naturally. If your PBN only ever links to your money site and nothing else, that's not how real sites behave.