For international students who went to US for undergrad, what is the fastest path to become a naturalized citizen without getting married to an American?
I am not an attorney but very familiar with the process. Being an undergrad puts you in an EB3 category. And you need to find a job within that category and an employer willing to file a green card for you. Depending on your country of birth, the entire process from day one to the day you receive a green card can take up to 2 years. More for people from certain countries of birth e.g. India/China.
That's a long conversation but the EB1 and EB2 green card categories allow for self-sponsorship although the standards are pretty high and generally require a track record of accomplishment. Alternatively, company sponsorship through the PERM process (in the EB2 or EB3 category) can be pretty quick (12-18 months) depending on the individual's country of nationality.