Is this with your own code or someone else's? It's always harder to understand someone else's code (at least at first). Everyone thinks and writes in a different way.

In either case, I think you could benefit from stepping through the code in a debugger. Depending on what the code is, give some data as input where you know/can guess what the output is. Using the debugger, step through each line to see what happens to the data. It can help break down long or complicated functions into simpler chunks.

Recognition of functions or snippets of code will come through repetition and exposure. Writing code helps more than reading as well. Even with all of that, it's still okay and common to have to look things up or review. I constantly have to check the syntax of C++ library functions, like snprintf, which I have used but not enough to memorize (and that's okay). Don't be discouraged. I've been in my career for 11 years now, around 9 of which is working with embedded C++, and I still feel like an imposter.