Two jobs, different fields
The name, the subtitle and the keyword field are what Apple reads to decide which searches you can appear in. A visitor barely notices the keyword field, because it is not shown to them at all.
The icon, the screenshots and the first lines of the description are what decides whether somebody who found you taps Get. They carry almost no search weight.
Write them as two separate jobs. A title stuffed with search terms that reads like a keyword list wins the search and loses the install, and you cannot tell which happened from a downloads number alone.
Name, subtitle and keywords get you found. Icon, screenshots and description get you installed.
The order to write them in
Start with the searches you want to win, because everything else follows from them. Then write the name and subtitle to carry the most important of those words while still reading like English. Then fill the keyword field with everything that did not fit.
Only then do the screenshots and the description, because now you know which promise brought the visitor here and the first screenshot can answer exactly that.
What you cannot change quickly
The name, subtitle, keyword field and screenshots are tied to a version submission, so changing them means a review. That is a real constraint on how fast you can iterate, and it is the reason to get the search terms right before you submit rather than after.
Promotional text is the exception. It sits above the description, is 170 characters, and can be changed at any time without submitting a new build. It is the one part of the page you can rewrite the day you think of something better.
Promotional text changes without a review. Everything else waits for one.
Localization multiplies the space
Each localization you add gets its own name, subtitle and keyword field, and Apple indexes them separately. Adding a second English localization is the most common way developers effectively double the keyword space they have, because the App Store treats the two sets as additive for a user in the right storefront.
This is worth doing carefully rather than quickly. A localization you fill with a machine translation reads badly to the people who speak that language, and they are the only people who will ever see it.