Do seo services include backlinks, if so are they just buying from others?

So I'm relatively new to SEO and I understand that backlinks are one of the most important aspects to improving rankings and building credibility. I've seen alot of businesses that sell backlinks and have heard mixed reviews about services like that. I've been trying to seo for my site myself and am at the point where I am considering buying backlinks to help with rankings. But I'm curious, do seo agencies include backlinks in their packages, and if so are they just buying backlinks from sites like rhino rank and passing them along to their clients or are there ways to get back links without buying them like that? I've heard people say to guest post and im just curious how one even goes about that? Is it literally as simple as emailing a site and asking to post some blogs? I'm sure this is a bit of a noob question but I've been wondering how backlinks work when I comes to agencies. Thanks in advance for any insights!