You can calculate 'Active Users' in GA4 BigQuery by counting the number of unique users who triggered at least one event during the specified date range that was flagged as active (is_active_user = true).
Once you understand the logic, calculating active users via text prompt is easy.
The SQL below is automatically generated via a text prompt in GA4 BigQuery Composer (a custom chatGPT) that calculates Active users in GA4 BigQuery: