We just added Bigquery as a data source, but we gave read-only permissions since we don't know where will Growthbook write the data. This is confusing tbh, even after reading this: https://docs.growthbook.io/guide/bigquery Will GB write data in the same data source? Doesn't it need to write data from the experiments? (like, which users were part of the variant vs control, etc?)

GrowthBook doesn't write to your warehouse using the SQL data connection. That is handled via the tracking callback you establish in your SDK, which will use whatever tracking tool (e.g. GA4) that you use for other trakcing. You should see some instructions there on the relevant SDK page.