Hi I think the answer to your question is that an idiom can be formed in English when referring to a number of outlets of the same business In other words, "Macdonald's" can be short for "Macdonald's restaurants" If I said - "In my high street, there are now two Macdonald's".. It might look as though I'm using apostrophe + s as a plural form to refer to two shops (which would be wrong). But the "'s" is not a plural marker - it is signifying "two Macdonald's shops" - two shops that belong to Macdonald Tricky to explain - but I hope this may help.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.