They are global collectives of individual country-level partnerships (though in some countries they are allowed to be corporate entities rather than partnership

They are global collectives of individual country-level partnerships (though in some countries they are allowed to be corporate entities rather than partnership entities). Each country-level partnership is its own independent firm, and can generally choose to re-affiliate with another multinational "firm" as it desires. This actually happens quite frequently. Generally, the member firms cross-bill each other at discounted rates. They make up for it through increased volume of work and/or more lucrative work.