Start out with an Arduino, or ESP32/ESP8266. The ESP boards are probably the best bang for your buck out there if you're just playing around— you can start out with the Arduino environment (C++-ish), or use something like PlatformIO to interface a bit more directly with the hardware. It's not as low level as an ARM board where you have to worry a bunch about setting up clock multipliers and all that jazz, but the power to do so is there when you're ready for it.

The best way to learn is to solve a problem that motivates you. Maybe you want a phone notification when your laundry is done, or when a room in your house exceeds a certain temperature. Work on some little projects like that

If you want to get even lower level, then try out the ARM boards again. Someone else mentioned STM32 boards, which are great in many ways but IMO not very user friendly (Their STMCube software actually makes me mad). You can also try your hand at FPGA development. Verilog and VHDL are both popular languages, and preferences between the two tend to depend on domain. There's compilers that let you program FPGAs in C, but shying away from that.

If you want to look for work, probably the big industries to check out are:

Consumer embedded hardware: companies that make printers, cell phone radios, etc

Medical devices

Aerospace/defense