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