The idea is simple. Mention the same entities and categories on your guest posts and on your site as well.
The idea is to strengthen relevance between your guest posts and the pages on your site, i.e. external site writes about entity A which links to your page that also mentions entity A.
This can work perfectly if your PBNs are in a different niche than your money sites, as well as with guest posts.
A tool that can help with this https://inlinks.net/
It takes the information from https://cloud.google.com/natural-language/ and presents it in a more digestible way.
If you have no clue what entities are, have a look at this article – https://www.oncrawl.com/technical-seo/introduction-entities-seo/