I've worked in book publishing most of my career, both with publishers and with independent authors. Trust me, the publisher's marketing strategy is nothing special.
The difference between a successful book and a middling book, barring the impossibly rare "so good it organically becomes a world beater," is the size of the author's platform. Publishers have a rigid PR/marketing process they run for every book, involving sending galleys (press copies) to the same press contacts, and sometimes those efforts net press coverage, and sometimes that coverage will result in some sales. If a press is very forward-thinking, by industry standards, they may invest in building the author a website or helping manage social.
However, books that hit bestseller lists are typically written by authors who have a large, engaged audience they've built on their own. The impact of a publisher's marketing budget is typically small, and definitely not worth giving up 80%-plus of your sales due to royalties.