Doing a lot of reading on ways to get the right keywords for a listing and be able to use tools to see which keywords are indexed by Amazon search. A good read is “A Guide to Amazon Indexing & How to Check Your Keyword Indexing” from searchenginejournal.com.
One of the tools mentioned is the KW Index and Rank Tracker Chrome extension from AMZDataStudio.
sellerapp.com has a good read in “The Ultimate 2021 guide to Amazon backend keywords“.
A Peek Behind the Curtain: Finding Amazon Backend Keywords
How Can I See What’s in the Backend of My Competitor’s Listing?
Amazon FBA Case Study: Spy On Your Competition & Steal Their Best Keywords– video with Helium 10 Bradley Sutton.