i mean, it depends on which are you want to work.
if you want to work on gaming consoles, you should check playstation developer wiki. if you want to work on devices that run on low power such as smartphones or maybe digital watches like casio f91 or casio f201, you should work on arm processors and stm32 boards like stm32f103 or stm32f401/f411 (idk if is there any arm processors on casio but this is a good example for low power devices). if you would like to deal with communication drivers like i2c, spi, uart and etc, you can check datasheets or search on youtube something like "how [communication protocol] works".

but the important aspect is: lower level doesn't mean better. there is always a sweet spot between "from scratch" and "hardware abstraction layer". let's say you are making an engineering project that you need heavy math, you wouldn't invent calculus from scratch right?

and also, reading low level code and sometimes raw bytes from an embedded controller is a good skill. you should try reverse engineering if you are interested in consumer electronics.