How do researchers send unsolicited emails asking for feedback on their works?
The real answer is that they don't. I'm sorry, but I can't slice it any other way. You would need all the planets to align in order to receive an answer after such an email. You are asking someone who doesn't know you and who is probably very busy (like all academics) to provide you a big favor for nothing in return. Because let's be honest, giving feedback on someone else's work is taxing. The best case scenario is that you get very vague and superficial feedback about the first page of your paper.

Instead, ask your colleagues, or people you have already built a rapport with (an advisor or former advisor, a collaborator...). Or do it the hard way and submit it for publication. You will get (anonymous) feedback. But make sure first that your paper is close to being ready for publication, because you don't want a desk rejection, and you don't want to be known as that person who wastes referees' time by submitting drafts.