Whats NDK (side-by-side) in android sdk?

This is how it looks like in my Android Studio 3.5 beta2:

You can see that I don't hide obsolete packages and request package details. Now we see that the old NDK package is replaced by the new NDK (Side by side), which allows to keep both NDK r19 and r20.

The reason why they are making the change is that switching to the latest version of NDK is not always painless, and many developers prefer to stay with an older version, at least for some projects.

The bottom line is that it is OK to use the NDK that you have installed, but it will be replaced with the new package some time later when you upgrade the Android Studio.

NDK (Side by side) is irrelevant for Android Gradle Plugin earlier than 3.5. However, the components available for download by SDK manager aren't customizable based on Android Gradle Plugin version so the side by side NDKs will appear.

Starting in 3.5, you can specify a particular version of NDK in your build.gradle by setting, for example:

android.ndkVersion '19.2.5345600'

The non side by side NDK has been marked as obsolete. Which means it won't appear in the SDK manager list unless you uncheck Hide obsolete packages --or-- you already have that NDK installed locally.

According to Android Studio 3.5 release document, it says: "Android Studio 3.5 allows you to use multiple versions of the Android NDK side-by-side in your build.gradle file. This should allow you to have more reproducible builds and mitigate incompatibilities between NDK versions and the Android gradle plugin." I think side by side means you may use multiple NDK versions simultaneously. It is a new version to replace the ndk bundle version.