What kind of apple fermented drink did I just make? I got this (dutch) recipe on Twitter a few months back. Translated it's called: apple with bubbles. I made it, and it was nice, simple process, and produced a nice summer style drink. But I was wondering, what did I make, and which parts can I try to vary with?
Translated recipe:

Put in a big jar: 1 apple (small chunks), 1 slice of lemon, 500ml water, 1 tsp honey, 20gr sugar, 1 tsp fennel seeds, 1 cinnamon stick
Cover jar with cheesecloth
For 3-4 days, put it somewhere without direct sunlight, stir/shake it twice a day. (So get some nice fermentation going)
Transfer/filter through cheesecloth liquid into a bottle
Leave the bottle outside the fridge for 2 days, make sure to releave it of pressure.
Put the bottle in the fridge for 2 days minimum. After that you have a week to drink it

So, uhm, what did I make, and which elements are essential for the process?
What you made is a watered  down version of a fermented or hard cider - the natural yeasts in the apple produce CO2 and alcohol, the sugar adds a bit of extra food for the yeasts, as does the honey which also contributes to the flavor, like the cinnamon stick and fennel.
In short, you used the spontaneous fermentation like it’s traditional for wine and cider, just with a diluted fruit mash.
For tweaking:
The sugar (or honey) is necessary as you added water to the apple juice. The spices are optional.
I currently have a batch of something quite similar on a shelf, where I first fermented apple peels and cores in water  (using up leftovers) and now am fermenting it a second time to make vinegar.