How to Determine Categories for Keyword Research?

This is something I struggle with and I'm curious if you guys use any tools for this or have a strategy you'd be willing to share.

Tools like SEMRush are great for validating exact match keyword target once you already have a topic in mind, but what's the most efficient way to generate topic ideas from a given category?

Example: a health site would want to find a bunch of relevant subcategories within the general category of health (e.g. "apple cider vinegar") to then plug into a keyword research tool like SEMRush. What's the best way to complete this step?