There’s definitely a correlation between paid ad traffic and organic website visits, starting with users visiting across multiple devices (found me with an ad on my mobile FB news feed, I’ll be back from my iPad or laptop). There’s also an inability to track some devices or users, that’s just a technical reality, though I do believe that Google maintains the capacity to accurately show you your “organic search” activity, it’s the “direct” and other nondescript sources in GA4 reports that’ll contain the rest.

Have you tested pulsing your ad campaigns, or even looked at the before and after where a client’s ad budget changed dramatically? There’s no single formula here, it takes some sleuthing, and be careful not to discount offline marketing activities as drivers of direct and organic site traffic. Still though, take a look and plot organic search and other/direct (sum of all unpaid that are not organic search) over time. Weekly is fine, give yourself a 2 or 3 month window pre- and post budget change. You’ll need to sprinkle some gut feel onto this, related to offline marketing, seasonality, and some other factors. Also, you don’t want to be doing this sort of thing incorrectly or you’ll jerk the wheel into a bridge abutment; find an expert, should you need one.