The first and most natural thing that comes to mind is asking yourself what would I type in if I was looking for my product? and this is a great place to start.

If you have completely separate topics, consider creating separate domains. In most cases, you’ll want to stick with a single domain, but there are times when multiple domains make sense. I’m thinking of starting a social media blog, and I’ll likely start it as its own domain rather than as a subfolder of my existing one.