Instagram Ads: How Does Facebook Determine User’s “Interests”?

I'm in the process of setting up Instagram Ads. I'm at the point where you type in "interests" for your audience.

I am very skeptical of this. I have no clue how well I can narrow down my audience using the "Interests" section.

Does Instagram make sure that at *at least* HALF of the interests on the list match the user before serving the ad? And where are these "interests" pulled from"? The content they engage with the most? From which part of a post do they pull the keywords to determine…

Instagram Ads: How Does Facebook Determine User's "Interests"?