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Found it. It is the function "site_aggregates_activity" which essentially says if you have published a comment or a post since the target time (day, month, 6 month etc) you are considered active. Here is the code

select c.creator_id from comment c inner join person u on c.creator_id = u.id where c.published > ('now'::timestamp - i::interval) and u.local = true union select p.creator_id from post p inner join person u on p.creator_id = u.id where p.published > ('now'::timestamp - i::interval) and u.local = true