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.