Link Building can be divided into 2 Ways!
1- Submission Links - Any link that is placed by you, doesn't need any approval or edit from the blog/site owner is a submission link. Example- Web 2.0 , Article submission, Social bookmark, Blog comment (auto approval), pdf submissions, press release submission (until a real journalist write about your blog) , PBN blogs etc etc.....
2- Outreached / Editorially Placed Link (Any link that required permission / edits of blog owner).
A- When you reach out to another blogger to place a link in his/her blog post to your article. It can be a real guest post, link placed within existing content etc.
It can be paid , free or for an exchange of something. You keep it secret. Google don't know about it.
B - When your article is out of the universe and so good to read. Other bloggers will give voluntary reference within their article. This is complete white hat link.
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Now get back to your question :-
A real guest posting site - Allows other bloggers to publish/contribute a content on his site, in return they get a link. May be in author bio box or in article. This is completely fine. Don't get too much worried about it.
A PBN Site - It's sole purpose is to place an out link in every article, doesn't matter where it is linking, the quality of target site etc.
Not all PBN are bad actually. 70% Affiliate sites are surviving over PBN links. If it is built on good quality and kept safe. It's like a gold mine.