Am I correct that the assignment into buckets is using a cookie with a random ID, and that the tracking in GA4 is using the user_pseudo_id , so there is no link between the assigment into buckets and the tracking to GA4. Growthbook can analyse the GA4 tracking by using the experiment_viewed grouped by user_pseudo_id, but will never be able to link it back to a "bucket cookie". Is this correct?

That is correct using our out of the box SDKs without any customization. The "no setup" config relies on the

gbuuid

we generate for the SDK and for randomization to be roughly 1:1 with the GA4

user_pseudo_id

as they both live in cookies.

So where does Growthbook then determine if a user_pseudo_id from GA4 was in a experiment variant A or B? This doens't seem to be in my fact tables

It's set up in your Datasource > Experiment Assignment Query. We normally auto-generate one for you if you tell us you're using GA4 and BigQuery.

Ah yes there I see it, now it makes sence 🙂

No problem!