STM32Fx Microcontroller Custom Bootloader Development

I was in your shoes a few months ago. I got my start with a few courses from Udemy. Particularly, the courses from FastBit Academy. They also have a good recommended order that makes sense. Let me list the courses below

1. Microcontroller Embedded C Programming: absolute beginners(Embedded C)
2. Embedded Systems Programming on ARM Cortex-M3/M4 Processor(ARM Cortex M4 Processor specific)
3. Mastering Microcontroller with Embedded Driver Development(MCU1)
4. Mastering Microcontroller: TIMERS, PWM, CAN, RTC,LOW POWER(MCU2)
5. Mastering RTOS: Hands-on FreeRTOS and STM32Fx with Debugging(RTOS)
6. ARM Cortex M Microcontroller DMA Programming Demystified(DMA)
7. STM32Fx Microcontroller Custom Bootloader Development(Bootloader)
8. Embedded Linux Step by Step using Beaglebone Black(Linux)

I highly recommend you check out this course on udemy. It helped me a lot when I was writing my first bootloader for the stm32.

https://www.udemy.com/course/stm32f4-arm-cortex-mx-custom-bootloader-development/

I'd recommend starting with a microcontroller since they're simpler and better documented than the Pi. The stm32 line from ST uses arm cores in their chips. Here's a course that's in C which would be a good starting point.

https://www.udemy.com/course/stm32f4-arm-cortex-mx-custom-bootloader-development/

No encryption, but checksum verification is included in this one:

https://www.udemy.com/course/stm32f4-arm-cortex-mx-custom-bootloader-development/