Side projects are a very good way to learn. Start with something very simple (even if you think ) like a simple terminal tic-tac-toe or a minesweeper. I insist, without a proper UI, without thinking of Android or iOS. It's much harder than you think now, and it will teach you a lot about code organization without having to worry about things specific to Android or iOS. Depending how fast you learn and how much time you have on your hands, this will already take you a week/a month/or more.
For modern apps, you will need to understand parallelization (e.g. coroutines). This could be as simple as making your tic-tac-toe properly multiplayer, but this is already more complicated than you'd think. If you're end goal is multiplatform code, I encourage you to use Ktor at that point, as you will be able to use it as-is later.
After that, it would be a good thing to find a simple idea of an Android-only app. After you're done with this, you will have enough knowledge to start thinking of multiplatform.