As I have said here on numerous occasions, success in the publishing world all comes down to sales. It doesn't matter how much your beta readers like your book, or for that matter, how much your agent and family loves it, if the sales are not there, don't expect to extend contracts with your publisher.
But, with that said sales depends on marketing.
Yes, your publisher is likely helping out a bit with marketing, but it will be your targeted approaches that will really drive those sales.
Now, when we say targeted, it comes down to knowing your readers and knowing their reading and purchasing habits. If you are just marketing the way you have seen your friends market their books, you may be missing out. Remember that not everyone buys things the same way.
Consider this. Let's assume you are writing something for an older audience. Does that audience heavily buy on the internet and social media? Do they follow Tik Tok? If not, that advertising is a complete waste on them. This might be an audience that would rely more on a newsletter approach. Of course, you have to get that letter into their hands, but once you do, they are more likely to follow all of your latest releases.
Another thing to consider is if your readers are digital readers or those that still love (like I do) a good book and the smell of paper and ink. Now, think about those contests that you might run to get some more business. Will they really compete? Probably not.
Finally, if you are someone with stronger international sales, or maybe your book sells better in certain regions of the US, then focus your attention there.
In the end, it really does come down to making sure that you are meeting the needs of YOUR readers. Not ALL readers, but those that love you and your books