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PROCESSOR-SDK-AM335X: where are kernel/u-boot source codes for modify&customizing my own linux system ?

Part Number: PROCESSOR-SDK-AM335X


Studying the TI-SDK, I found yoctoproject  is a powerful tool.
but still i don't know detail feature about it.
my build procedure is ...
1. install TI-SDK (download from TI hompage)
2. install ARM toolchain
3. clone git repository ( git clone git://arago-project.org/git/projects/oe-layersetup.git tisdk )
4. and then follow TI-SDK quick start guide ..

cd tisdk

./oe-layertool-setup.sh -f configs/processor-sdk/processor-sdk-06.00.00.07-config.txt
cd build

. conf/setenv
export TOOLCHAIN_PATH_ARMV7=$HOME/gcc-arm-8.3-2019.03-x86_64-arm-linux-gnueabihf
export TOOLCHAIN_PATH_ARMV8=$HOME/gcc-arm-8.3-2019.03-x86_64-aarch64-linux-gnu
MACHINE=am335x-evm bitbake arago-base-tisdk-image

and i successfully build beaglebone black image and run.
So, my question is
1. where is downloaded (or copied from TI-SDK) u-boot/kernel source code for customizing my own linux image ?
    i want import new device driver, add read new device tree mechanism, apply patch for realtime linux and so on.
    there is too much directories and TI-SDK quick start guide don't tell me where it is. i can't found it.
2. i download and install TI-SDK from TI website. but what is the my build procedure "step 3 : clone git repository ( git clone git://arago-project.org/git/projects/oe-layersetup.git tisdk ) " ?
    this is also included TI-SDK ? ( i think this is yocto project repo. may be.)
3. my first build spends a lot of time (about 8 or more hours) .
    If I rebuild later, it will end soon ?