On the subject of levels: 40Watts brings up a good point to start learning that right away. Every engine has some sort of system for building levels and for transitioning from level to level.
Another point about levels is to get comfortable with how levels work in your engine of choice (or “scenes” as some engines call them). At first you may just start building out every level. But once you learn your way around, you can start to sort of template your levels. I make, at most, four levels per game. The main playable levels will read in some sort of configuration that tells it what assets to load and what to do with those assets.
For 2D games, for example, you could use something like Tiled to build your levels (https://thorbjorn.itch.io/tiled). So you build one level in your game, then just use Tiled to drag&drop levels all day long :)