Like any niche, it's hard to know where to start and it also depends if you are more interested in HW design, or the firmware side of things. You need some knowledge of both since they overlap in many areas in embedded, but they are different paths.

Assuming you mean more writing firmware, the biggest thing to understand is that embedded is all about C. You'll absolutely want to learn the basics of C and properly understand pointers. A key part of C is understanding data types (signed, unsigned, float) and notations you rarely used in other fields like hexadecimal which is omnipresent in embedded. If you grew up learning C#, node, etc., you likely don't properly appreciate these fundamental types, and you'll need to learn those fundamentals, but that will come with learning C.

For books, I like Jack Ganssle's "The Art of Designing Embedded Systems". He does a good job of laying a solid foundation for planning embedded projects. It's opinionated, but you could do worse than start with his ideas.

And start with a professionally maintained foundation for your projects. Arduino is good for some people, but it won't scale and won't give you the skills you need professionally, and scripting languages like MicroPython won't help you later in life. Use a language (C) and platform you can go to production with, such as Zephyr RTOS, Azure RTOS, FreeRTOS, etc. It's more work and harder up front, but the investment will pay dividends and you'll learn good habits from the start.