proper nouns - Possessive function of a business name which is made with a possessive

Garner's fourth edition, page 714, states regarding the name McDonald’s

It is quite defensible to write McDonald’s dinner combos (the name
functioning as a kind of possessive)

On what grounds can a business name which is made with a possessive function as a (kind of) possessive?

Secondly, is the same true for a business name created with a plural noun, therefore ending in -s'? (I am not a native speaker, and unfortunately cannot come up with an example right now)