For years I created many different apps in any niche I found. My idea was building these apps very quickly but lightweight, so I created my own WebView/JS interface instead of using Phonegap/React/Electron. Received lots of spam from ads/marketing/design companies, youtubers, and others. Last year got one of these emails from an affiliate marketing company, but seemed honest. I tried it out and for the first few months didn't receive a penny. I just forgot about them, but suddendly started receiving $200-$500/month. Guess these actually work if they match your niche.