Posting papers to arXiv is up to the discretion of the authors. Undoubtedly, there are papers which are peer reviewed that do not have preprints posted to the arXiv. Often people will post to arXiv as the papers are going through the peer review process. This enables the community to provide feedback in addition to the peer review process. Sometimes people do not post papers to arXiv until they have already completed the peer review process, but before the paper is actually published in a journal. And, less frequently, people will post past manuscripts to the arXiv if they think the community would benefit from having easier access to that piece of work (e.g., a chapter in a book that has gone out of print, for which the author still retains copyright).
I am speaking mostly from personal experience here, based on my interactions with, and postings to the astrophysics portion of arXiv[0]. Norms may differ in other fields using arXiv.
My understanding is being published on arxiv.org does not mean it is currently in a scientific journal. Being in a scientific journal does not mean it is on arxiv.org.
But, being published on arxiv.org is usually a precursor to the journal publication, passing peer review and the journal 's standards.