A sitemap is a discovery hint, not a ranking lever. Two of its five fields have been ignored by Google for years, one only helps if you are honest about it, and the limits are stricter than most people realize. Here is how to build one that earns its keep.
A Disallow rule tells crawlers not to fetch a URL. It does not tell them to keep that URL out of search results, and those two things are not the same. Here is the difference, the directives Google honors, the ones it quietly ignores, and the mistake that puts a blocked page in the index anyway.