Where are ${EXECUTABLE_NAME} and ${PRODUCT_NAME} defined

The PRODUCT_NAME is defined in the target's Build Settings in the Packaging section. It has the same name by default as your project.

Edit:

While PRODUCT_NAME is by default the name of the Target (MyDemoApp in this case). The EXECUTABLE_NAME is a concatenation of:

$EXECUTABLE_PREFIX, $PRODUCT_NAME and $EXECUTABLE_SUFFIX.

See the reference of EXECUTABLE_NAME for details.

Update

The new reference can be found here http://help.apple.com/xcode/mac/8.3/#/itcaec37c2a6

EXECUTABLE_NAME
Specifies the name of the binary the target produces.

I think the $ represents the variable productName & executableName as the $ is used in unix. The variables are set in the build setting of the application. So you should not change the product name directly in the plist file. Instead, go to build settings, search for product name and change it.