PostgreSQL Internal

create table tab (id int, name char(16));
insert into tab values (1, 'abc');
insert into tab values (2, 'def');

select string_agg(to_char(id,'999'), ','), string_agg(name, ',') from tab;

postgres runtime function call trace, click each function to explore
PostgresMain 
  exec_simple_query
    ...
    PortalRun
      ...
      advance_aggregates
        ...
          int4_to_char