Conventional trade publishers, those who take on all the expense of editing and producing a book on behalf of the author, are (understandably) extremely selective and will take on only those books which they are confident will sell enough copies to make money, usually from established authors. They will rarely deal with an author who does not have an agent (and agents are just as choosy about which authors they take on). If a trade publisher does agree to publish your book, it will exercise control over the content, text, format and cover design and will naturally expect to profit from sales, allowing you only a modest royalty. It will also retain a share of the rights.
Beware of the many companies which suggest on their websites that they will publish your book free, but in fact once they have ‘reviewed’ it, they will come back to you asking for a substantial contribution. They know that by the time they ask you for money, you’ll feel engaged with them and flattered by their comments, and you’ll probably agree to pay up.