Why are data files getting modified?

Each partition has about 2-3 billion rows and is about 40TB of data in total.

I am running a select query to \copy the data into csv files for each month.

I see there is an AUTOVACUUM job started on all the partitions that have been read (on which select has run). I am assuming the reason why Postgres is autovacuuming these large partitions is this:

"This implies that if a table is not otherwise vacuumed, autovacuum will be invoked on it approximately once every autovacuum_freeze_max_age minus vacuum_freeze_min_age transactions. For tables that are regularly vacuumed for space reclamation purposes, this is of little importance. However, for static tables (including tables that receive inserts, but no updates or deletes), there is no need to vacuum for space reclamation, so it can be useful to try to maximize the interval between forced autovacuums on very large static tables. Obviously one can do this either by increasing autovacuum_freeze_max_age or decreasing vacuum_freeze_min_age."

db=# SELECT c.relname as table_name, c.relkind as type, age(c.relfrozenxid) as age, c.relfrozenxid FROM pg_class AS c WHERE age(c.relfrozenxid) <> 2147483647 ORDER BY 3 DESC LIMIT 20;
        table_name         | type |    age    | relfrozenxid
---------------------------+------+-----------+--------------
 hist_omx_orderaudit201506 | r    | 201463508 |       319891
 hist_omx_orderaudit201509 | r    | 201418497 |       364902
 hist_omx_orderaudit201205 | r    | 201142042 |       641357

autovacuum_freeze_max_age
---------------------------
 200000000
(1 row)

Am I wrong in my understanding why this autovacuum is getting triggered?

However, my main question is that why do the data files in the tablespace, corresponding to these partition tables (that have been read or selected recently), or the ones autovacuumed recently have a latest timestamp?

There was no insert or update on these partitions, so why do I see an updated timestamp? Does autovacuum change something on the datafiles?

Thanks