Specifically how is where clothes are manufactured determined?

I have been shopping at Gap for nearly a decade, on and off (I went through a several year period where I refused to buy clothes made in sweatshops and did not buy anything from them, or pretty much anyone else, and when I started looking like a hobo I had to relent).

I have a long sleeve shirt made in Greece.
Another one made in Vietnam.
Panties made in Sri Lanka.
Tanktop made in Israel.
Bra made in Sri Lanka.
Other bra made in Israel.

Obviously economics is driving this, and it is not unique to Gap. Does anyone know how they determine where something will be made?

I just bought a shirt for $2.99. Obviously someone was exploited for that. Has anyone made a documentary about this?