Why are data files getting modified?

I have a table with monthly partitions, starting from 2011.
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."

https://www.postgresql.org/docs/13/routine-vacuuming.html#VACUUM-FOR-WRAPAROUND

Please see these partition tables are static, i.e., they received data many
years ago, with no updates ever.

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

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