Hi there, glad to hear you’re enjoying coding!
There’s a few ways you can go about doing it. At the end of most of the lessons you are presented with an empty environment on which you can practise, you’ll find this as you go along as well, some come with pre-made CSS and HTML templates for you to mess with. If you really want to get building from complete scratch, then I recommend getting set up with VS Code. It’s a lovely text editor to use, with a ton of helpful auto-complete features you can install. That way you can hold all the coding files on your own PC, and build whatever you want from scratch!
I have quite an extensive library now for all my projects, and it can be really helpful when as a database of sorts for code that you can quickly refer back to if you wish to reuse elements.
Alternatively you can also use something like repl.it, which is an entirely free browser based coder for practising. It works much like Codecademy’s editor but gives you the freedom of being outside of any sort of lesson. VS Code is worth setting up in the long run if you plan on coding for a while, but this can be a nice quick solution for testing code before that’s set up.