In fact, it should be a member of your team, a staff writer. After all, it will be best of all, he will better understand the features of the product and understand your goals.

There is a second option, find a freelance writer, or even entrust it to writing services https://myonlineessay.com/ but this option is worse than the first

I'm working in writing company and usually, our clients are students who order some college papers for example like this https://familyessay.org/research-papers/ but I also know that often site owners ask us for writing help for their products. I'm sure that it has own place because not everyone can write well about something. But if you think that you are an expert in your niche and you know your product and audience well you should write it yourself because it is the best way for your business.

Me. I write them all, with lots of feedback from other people on the team. Some are written prior to the product launch. Some are written as a response to common support questions.