You can write databases and other things in java/scala and people sometimes do so (though often use lower level languages for more control), but to do it effici

You can write databases and other things in java/scala and people sometimes do so (though often use lower level languages for more control), but to do it efficiently you'll need deeper understanding of how whole stack works, including (at least) how it interfaces with operating system, how memory is organized and managed and what os and hardware can/can't do for you. I suggest to pick some project that interests you and get familiar with how it works, why it does so and how you could contribute. C/C++ are popular choices, though recently I am learning rust and that brings a tons of low-level knowledge with nice safety net (c is a language that preys on your mistakes, rust will not let you compile them).