Need Help Keyword in Domain Name Question

I need help. I see sites with main domain names not relevant to to keyword of product they are trying to rank for. For fictitious example lets say you want to market Grinnell Grand Piano( Or other brand names).The main product keyword is Grinnell Grand Piano Yet the main domain site is a review site and is called Mike's Music Review with multiple brands reviewed. So that means you have mikesmusicreview.com/grinnellgrandpiano as a web page. How does that person get the main domain to the top of Googles listings for a search of Grinnell Grand piano. I thought for best seo practice you wanted to choose a domain name with the keyword before the dot com. As in grinnellgrandpiano.com. I realize you can use different brands in the review. I thought to get Mikes Music Review to the top of Google you had to write articles with Mikes Music Review as the link. How does writing supporting articles on Grinnell Grand Pianos(or multiple other brands) get Mike Music Review to the top of Google. I know it is done. I see it in my searches that is the reason of my question How? Please help me understand how this works. Then what is the strategy for setting up a site. How does one pick and settle on "Mikes Music Review"? Just confused about how to put the cart before the horse.