- views_handler_argument_many_to_one::query in core/
modules/ views/ handlers/ views_handler_argument_many_to_one.inc - Set up the query for this argument.
- views_handler_filter_many_to_one::ensure_my_table in core/
modules/ views/ handlers/ views_handler_filter_many_to_one.inc - Override ensure_my_table so we can control how this joins in. The operator actually has influence over joining.