It depends on a lot of factors.

Big publishers will have direct accounts with bookstores. But you also have to factor in how much effort they’re putting into each book. Obviously, they’ll push big authors and titles they feel will sell. They can’t do that (and won’t do that) for every single author.

Go to a bookstore and look at the shelves and shelves of books. How many of those got prime distribution? How many were placed face-out and marketed to the extent that you knew they existed? And you’re always battling the incumbents (e.g. Stephen King, Lee Child, Di Morrissey, Diana Galbadon) and celebrity authors for prime real estate. Just because a publisher gets your book in a bookstore doesn’t mean it’s going to thrive. It could sit on some lone bottom shelf for months.

Other publishers use distributors that the self-publishing author also has access to.

Now, also, using print-on-demand a book can be distributed globally and appear in databases for offline and physical retailers. This doesn’t mean the physical retailer will stock it. Big bookstore chains generally won’t deal with individuals. Independent bookstores often will.

Ultimately, while distribution plays an important part, it still comes down to marketing. Who knows your book exists? You can have ten copies in a bookstore — it’s not going to matter unless people are driven there to buy it.

A big publisher will place finite marketing into your book before they need to move onto whatever’s next on their publishing slate.

The onus always falls on the author to create awareness for their book and themselves.