Are you more interested in understanding the processor itself, or low-level software, eg operating systems concepts and how that interacts with the hardware?

Embedded can certainly help with both, but I would go in the opposite direction from hobbyist-oriented systems like Arduino: learn to set everything up yourself. Bare metal programming, particularly with C and assembly, will require you to understand things like stack pointers, setting up your clocks, initialising hardware.

While 8-bit micros are simpler and you could start with one of those if you wanted (AVR is probably the most accessible), I'd probably go straight to a 32-bit Cortex-M4, get a cheap dev board like [0], and get the sample programs running and try to understand every part of those.

[0] https://www.st.com/en/evaluation-tools/nucleo-f401re.html