SQL Server default installation settings is to install the SQL Server Browser service as disabled (not running)
CB bootstrap installer only detects existing instances of SQL Server if the SQL Server Browser service is running
Defect
If you choose the install option "Use an existing installation of MS SQL Server on this local computer" (see screen clip 1), and if the CB bootstrap installer does not detect an existing instance of SQL Server (aka there is an instance installed but the browser service is not running), the CB bootstrap installer ignores your selected option "use an existing installation..." and proceeds to install a new and unwanted instance of SQL Server Express.
It is not so bad to have this happen in a test environment. However, it can be damaging to a customers production SQL Server environment to have unwanted software installed without permission and can waste much time if this causes problems and/or needs to be uninstalled.
Factors
Defect If you choose the install option "Use an existing installation of MS SQL Server on this local computer" (see screen clip 1), and if the CB bootstrap installer does not detect an existing instance of SQL Server (aka there is an instance installed but the browser service is not running), the CB bootstrap installer ignores your selected option "use an existing installation..." and proceeds to install a new and unwanted instance of SQL Server Express.
It is not so bad to have this happen in a test environment. However, it can be damaging to a customers production SQL Server environment to have unwanted software installed without permission and can waste much time if this causes problems and/or needs to be uninstalled.
VIDEO RECORDING OF ISSUE https://user-images.githubusercontent.com/453837/104452793-5961d980-5558-11eb-9370-e24df3348342.mp4
SCREEN CLIP 1: