All Articles mentioned below have corresponding Youtube Video link as well.
Linux
Here are links to start with linux , important topic like busy-box and bootloader
Utilizing QEMU to understand Firmware and Bootloader
STM32
STM32 specific important links covering memory map and boot related information.
RISC -V
Next Generation Programming with RISC -V
Programming Concepts with RISC-V
Learn C
Embedded C data Structure A practical Approach
C Learning Tips with Embedded Architecture
Embedded Career