[curiousfiddler]

There are quite a few, and you can create a list of categories you consider as fundamentals for the nature of your work. As an example, I would think Algorithms and Data Structures is a fundamental subject. These are the easiest to practice.

You could for example pick something as simple as a HashTable, and implement it from scratch. Then, you could add more complexity to it, like HTs that won't fit in memory, expanding and shrinking HTs efficiently etc.

Or, you could use one of the several practice websites like LeetCode to practice Algorithms/DS problems.

Once you start building a habit, you will also become better at organizing your practice routine and finding out more about what to work on, and where to look for study/practice materials. But mind you, this is a slow process, which you want to build as a habit. There is no end goal here (like cracking Google interview or such), this is a process to get better at the fundamental skills in your field.