new user here on Stackoverflow but i can sure answer your question permanently and fully.
Flutter has a default NDK version which it uses for its projects, doesnt matter you have it in your system or not.

If its not in your system and even if a higher NDK version is present, it will try to download the default version
The location of default version for kotlin flutter is
Your_flutter_SDK\packages\flutter_tools\gradle\src\main\kotlin\FlutterExtension.kt`

in here go to line which looks like this, version might be different , search for ndkVersion

val ndkVersion: String = "29.0.13113456"

change it to the highest version available on Android studio SDK Manager , and download the same in SDK manager , since they are backwards compatible, so it is okey.
Now any further projects you create on flutter will use this ndk and you wont have to manually change ndk version in build.gradle file in every project manually.