PBNs are simply a collection of websites you own, that are designed for a wide variety of niches. From a visual persepctive, it should be impossible to tell if any of the sites have a relationship with each other. Think “credit/apartment sites” + “cooking recipes” + “coding school reviews”….
There’s actually a white-hat way to monopolize sites like these. Just look at online publisher tycoons like mediabistro or eCommerce kingpin Amazon’s (Diapers.com, Saop, Wag, Yoyo, Beauty Bar, Casa, Vine market, etc….). These are perfect examples of how a good PBN works, except these examples are public.
For the purposes of SEO, your PBN can be used to specifically “boost” a site with links. Every black-hat offering link building services has their own PBN which they use as a business engine.
You can go about it which ever way is more convenient for you. This is just a primer and there are plenty of in-depth guides out there detailing how to automate each site with copy-free content, etc.