Providers of fake DOIs?
DOIs are registered and managed by Registration Agencies appointed by the International DOI Foundation. All registered DOI numbers resolve through https://doi.org/ - if it doesn't resolve it's not a DOI. A DOI can be applied to many different things so the fact that content has a DOI doesn't say anything about the quality or validity of the content. There's a blog post about this - https://www.crossref.org/blog/dois-unambiguously-and-persistently-identify-published-trustworthy-citable-online-scholarly-literature-right/