Working with iOS app keywords
is very different from Google Store. Here you have a small field of 100 characters where you need to fit the most relevant keys. Keep in mind that this store does not have such powerful search tools as Google, and therefore does not take into account service words, is unable to pick synonyms, and does not distinguish word forms. If you have multiple similar keywords, you need to list them all (synonyms, plural forms, etc.). Of course, 100 characters are not enough to mention all the terms; pick their most relevant forms and list them separated by commas (no spaces).
Also, keep in mind that you don't need to use the same keyword in several places when doing iOS app store optimization. Internal index algorithms will only take into account the mention that has the highest weight. Don't write phrases consisting of more than two words. The store's tools identify single words and combine them as they see fit. The total number of combinations can reach tens of thousands. In general, the system does not work well with structures longer than three words; the best size is one or two ones.
The text in the keywords field is not visible to users, so some developers try to include the names of major competing brands. It can really lift the product in the search results, but you'll be blocked if moderators find that you have gone for such a trick. Choose relevant keywords carefully because you can only change metadata on App Store when you upgrade the product. To edit only a keyword, an iOS app update is unacceptable; you should make significant changes to the item itself.