"The fundamental idea of the MWI, going back to Everett 1957, is that there are myriads of worlds in the Universe in addition to the world we are aware of," it says here in that link you gave us so obligingly above.

But why doesn't Mr Everett then go on to answer the vital question? ~~

viz ~ Are they all mounted on the back of the same Giant Tortoise, or do they each have a separate individual Giant Tortoise of their own?

I think we should be told!