The new Nokia phones are engineered and designed by a company called HMD. HMD was founded for the specific purpose of making "Nokia" phones under the license after Microsoft sold it off. HMD is based in Espoo, with their office right across the street from Nokia HQ, and the company consists mostly of ex-Nokia phone engineers and other employees.

So basically the phones are designed by the same engineers, and the office where they are designed is right next-door to Nokia. The reason it's a separate company is just that Nokia is not that interested in making phones right now.