Your publisher advances you roughly the amount they think the book will make in royalties, and then they pay for the editing, the formatting, the printing, some promo, etc. They have in-house teams of marketers and publicists who will at LEAST pitch your book to libraries, retailers, etc.
They may also pay for travel, events, etc.; increasingly, they HAVE become over-reliant on authors taking on a lot of promo work/costs themselves. My hunch is that because readers are increasingly relying on influencers to find new books, publishers are struggling to market in this environment...
... and thus authors who can cultivate a following and *become* the product more than their books are, for good or for ill, moving a lot of copies. That's a whole separate thread, though. And it's not something you escape by going self pub either imho.
tl;dr With self-pub, you *will* take on all the significant associated publishing costs up front, but take home all the proceeds... which MAY net a profit. With trad pub, you will receive a lower share of the profit, but the only up front cost is your time, and you benefit from a publisher's reach.