Much of my job is embedded engineering, to establish a bit of credulity for the following recommendations. My employer is partial to ST Micro and Renesas, so keep that in mind. Others may suggest Silicon Labs, Microchip, or Texas Instruments. This will also be biased by my own education and work experience (e.g. no robotics, more sensors/lights stuff).

I'd recommend getting a dev kit like the STM32F4DISCOVERY (https://www.st.com/en/evaluation-tools/stm32f4discovery.html). ST Micro's boards are often used for courses (https://www.udemy.com/course/cortex-m/) so you may like to take some of those courses. You'll often hear about the TI MSP430 as another microcontroller but AFAIK it's beginning to be a bit dated. Although come to think of it, there's probably more educational material out there for it, if you're willing to search.

Grab a kit like the Sparkfun Beginner's Kit (https://www.sparkfun.com/products/13973) and read some of the tutorials on their website about creating circuits. Tutorials or courses for your dev kit should get you to a point where you can light an LED controlled by the micro.

From there, you may like to do more advanced stuff like communicating with sensors over specific protocols (Sparkfun's Tinker Kit and associated guides may be of use https://www.sparkfun.com/products/14556 though you will have to translate from Arduino to C code, which can be good practice for knowing how Arduino works under-the-hood).

At this point, you'll probably know whether you want to keep learning more about sensors/lights/IoT type stuff, or want to branch out to other embedded-related topics. More advanced IoT material will be things like taking sensor measurements, storing measurements to memory, interfacing with displays, sending data via WiFi or Bluetooth.

Edit: I skimmed over a lot to keep it short. There's a lot hiding behind how casually these recommendations are made, so feel free to reach out with any questions (email in profile).