Keep in mind that employers will usually not be motivated to provide you feedback once they have rejected you - as far as they are concerned, the relationship is over. This isn't as malicious as it may sound, it's just business.
In fact, they may be motivated to not give you feedback - even innocent-sounding feedback can be twisted into discrimination lawsuits, and although this may not be incredibly common, it's a risk that employers typically protect themselves from by not providing feedback. Another dimension of risk-aversion is that the feedback may come off as insulting or brutal, if they were honest. How many people like to hear that they're over-confident, or can't communicate simple ideas, or other soft skills gaps? While you may want that kind of feedback, many people would just be angry about it, and that's a risk for the employer.
It's also important to recognize that they may not actually have anything substantial to say in terms of actionably differentiating you from other candidates. Interviews are (usually) about selecting one person for one role. Not being selected doesn't mean you were a bad fit. It just means that there was someone else who was a better fit. Sometimes the difference is incredibly small and often it's intangible and hard to describe. This may seem unfair, but ultimately the process needs to be practical.
All that said, you have little to lose by asking, and occasionally an employer may actually respond - especially if they liked you (but not for that specific role) and may want to keep you in mind for future roles. You can model your question on that ideal scenario:
Hi Ms. Recruiter,
I want to thank you again for taking the time to interview me for the Software Engineer job. I understand that Acme Co. is very selective in their hiring process, and I'd hope to be considered for future roles with Acme. On that note, I was wondering if you would be able to provide me with any feedback on areas I could improve to help my chances of being hired by Acme in the future?
Thanks Again, wilson zhang
I'm editing to add another dimension to this. You did not mention working with a third party recruiter, but if you choose a recruiter who is well-liked and respected by the types of employers you're going after, this is an area where they can really help. Companies are often more willing to give specific feedback to a recruiter, who can then appropriately filter and communicate that feedback to candidates, and help steer candidates towards opportunities where they would be a better fit. Of course, this is an ideal case, and not all recruiters will be this helpful.