Hello @parth kothiya ,
ST does not provide the Bootloader source code.
To develop/use code of Bootloader, you can use a similar package called OpenBootloader (OpenBL) that behaves exactly like the system bootloader and uses same interfaces but is open source code.
OpenBL supports only STM32G0 and STM32G4 series for the moment. So, you can customize it for other products series according to your needs.
\STM32Cube_FW_G4_V1.4.0\Middlewares\ST\OpenBootloader
\STM32Cube_FW_G4_V1.4.0\Projects\NUCLEO-G491RE\Applications\OpenBootloader
More information on OpenBL is available for example in USART protocol used in the STM32 bootloader (AN3155).
Note that ST OpenBL Wiki page link will be available soon.
Hope my answer helped you! :smiling_face_with_smiling_eyes:
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Imen
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