Label on my phone charger says it's output is DC 5V and 2.1mA. I find this confusing, because at school I was thought that you can only talk about current when you know exact voltage AND RESISTANCE. So, using Ohms law, I thinks manufacturer assumes that resistance of my battery is exactly 5/2.1=2.38 Ohms.
But how can they assume that if people might use this charger for different types of batteries? Why don't they simply say it's 5V?
That's not what that means.
A current rating of 2.1A means that the charger can support a max current of 2.1A. But it doesn't mean that it needs to output that at all times. It can just as easily output 1A or 0.5A.
Some chargers have a minimal output current required to stay active (like battery packs sometimes) or they shut down usually on the order of 50mA.
It is true, you can't say if a power supply/charger can "give" 2.1 A of current without loading it with a resistive load. What the manufacturer is telling you is that this charger won't be able to drive a load less than 2.38 Ohms (which corresponds to 2.1A as you calculated). If you load it more, either its voltage will drop down (as chargers do), or it will completely cut the output off.
As a matter of fact, your battery (or phone or whatever) doesn't have any definite resistance. There is an actual battery charger inside your phone/tablet that regulates the current intake. So the load might look like a 2.38 ohm from the Ohm's Law perspective, and the "effective" resistance will only go up (drawing less current) as the battery charge approaches the fully-charged state.
