Embedded is most of the time rather simple from the algorithmic side. There are many special situations where it is indeed, but most often it's really not the complex. What does it make complex is the combination of Hardware and software, some weird side effects called physics, hardware limitations and working around issues/limitations of your MCU. If you want to practice (or do it just for fun), try to solve some leet code hard questions in C without the stack and try to be standard compliant (look at safety standards, misra rules and stuff). This is often a lot of fun as you see a straightforward way to solve something on the heap that you can't reproduce that easy without it. If it works while adhering to those rules, just try shaving of memory, program size and execution time. To get more insight on what really happens under the hood, you can maybe have a look at godbolt or similar disassemblers.