I am not from NYC nor do I have experience with this situation but, I do know a big difference (certainly in price) is education. A designer may or may not have a college degree. An architect will have a college degree plus, in all likelyhood, a post graduate degree such as a master’s degree or phD. They may have multiple degrees including an MBA. They will also likely be required to be licensed with the state which may come with mandatory certifications that often have to be renewed through more education on a semi-regular basis. That is why designers are generally more affordable.
I would not be surprised if you hire a designer you will also need an architect and/or structural engineer. Check the credentials of the designers you talk to.