Backlinks (New to Digital Marketing)

Hi everyone. I'm very new to digital marketing and have been studying everything from SEO, to Paid Ads, to CRM, etc.

One thing that has stumped me lately is gigs on places like Fiverr, where people are selling services for 1000+ backlinks. From everything I have learned so far, I don't see how that's possible. At least not in "white hat" terms. Are those gigs something that would screw a page / creator over if they are looking to get ranked or pull in organic follows either on YouTube, Google, etc? I want to, at some point, pull in some work by doing SEO for people, but don't want to screw them over if that's something I end up learning).

Thanks in advance! I can't really find anything on it on my own.