My two cents: almost all my sales are from other authors that I personally connected with. Chatted. Bought their book. Wrote a review. Basically, it’s that formula, rinse/repeat. I’m honestly more interested in getting eyes on my page and letting my book sell itself. So my campaign is mostly going to be paying for ads and being confident readers will love what they see.
Heck, I can’t even get my siblings to buy my book. So what do I know.