How do I set the default properties in an AOSP build?

There are two main places I usually check for defaults: /frameworks/base/packages/SettingsProvider/res/values/defaults.xml and: /frameworks/base/core/res/res/values/config.xml Its also worth checking the device directory for overlays which just override the original files: /device/<company-name>/<product-name>/overlay/frameworks/base/core/res/res/values/config.xml Default wallpaper usually hardcoded to: /frameworks/base/core/res/res/drawable-nodpi/default_wallpaper.jpg Default volume levels for various streams(int[] DEFAULT_STREAM_VOLUME): /frameworks/base/media/java/android/media/

What is boot.img file in android?

boot.img contains the kernel and ramdisk, critical files necessary to load the device before the filesystem can be mounted. You have to generate the boot.img yourself using mkbootimg, a tool provided by AOSP. All the details you need are available at this xda-developers thread. This google discussion thread may also be useful