Google Analytics support page does not provide specifics on how new vs returning users are defined. https://support.google.com/analytics/answer/1012034?hl=en#Behavior
ga.js and analytics.js user tracking. The first time a user visits your site, a _ga cookie is set to help distinguish the user. The cookie has a 2 year expiration date from the time it is set.
If the user visits your site within the 2 year period, they will be marked as a returning visitor. On the return visit, the expiration date for the cookie will be updated to be 2 years from the latest visit.
User clears cookies. If a user clears their cookies, they will be marked as a new user on their next visit.
Using the New vs Returning reports will help you realize the potential for different types of visitors to your website and how you can best make the most of your traffic. Adding custom variables will take this analysis to the next level and really make the data matter more to you.