Sorry, it's my friend who does it, not me, and I can tell you right now his clients are dumb finance/business bros, not tech.
All he does is throw the job description into some version of chatGPT to generate a cover letter, and edit it around to make it relevant to the person's skills and sound like a human wrote it. Then he reduces the resume down to one page. Usually this is enough to get them into the group interview (according to him).
Basic stuff to tech oriented people but this shit is still a mystery to so many college kids/recent grads.