What does these versions name in AOSP source tree means?

I am building a custom rom from AOSP source code

I first decided to edit some AOSP apps from AOSP source tree by downloading and importing to Android Studio

I selected ExactCalculator for training

But I confused by confusing branch names in AOSP tree . I tried downloading pie release version of ExactCalcuator but can't find which Pie branch is latest

Below is the screenshot ::
https://android.googlesource.com/platform/packages/apps/ExactCalculator/+refs

I can't understand what is this ... b4s4,cts,cuttlefish,dr1,gsi,grpc,platform release.... etc

To be precise , I need the latest version of Pie running in our Android phones

Please make me to understand this branches of a release ....

You would pick a branch or tag for the whole AOSP and use the corresponding branch/tag in the ExactCalculator sub-repository as well.

repo init -u https://android.googlesource.com/platform/manifest -b pie-release

The .repo/manifest.xml (a symlink to default.xml) determines which branch or tag is used in ExactCalculator. Since the default.xml does not specify a different branch/tag name for ExactCalculator the default one is being used, which is pie-release as well at the moment.

Please make me to understand this branches of a release ....

I think you need to contact Google directly for clarification of the branch meanings. I suspect that it is just different teams working on different branches.

To be precise , I need the latest version of Pie running in our Android phones

I use pie-release for the latest release version and pie-dev for the latest development version.