Sponsored pages and brand mentions

I'm just getting started with SEO and backlinking. Many advertisers are now interested in publishing their links on my site.

What is the impact of allowing promotional links in my articles? What is the impact of allowing brand mentions? Should the entire article always be listed as "sponsored"? How does Google distinguish me mentioning a brand from an advertiser wanting to promote their brand?

What is the impact of linking to an unrelated topic in a blog post? In other words, if my topic is on breakfast cereal and I link to a blog about unhealthy foods, how does that affect my ranking?