How does it draw the exact current it needs? Is it as simple as that because my headphones just have resistance of 5 ohm?
And also, my 5V2A charger can charge any 5V devices that draw current equal to or below 2A right?
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Voltage, resistance, and amperage (current) are directly interlinked with a simple equation (V = IR, where I is current because reasons). For this question, all that really means is that if you lock down two of these to specific values, then the third one must also become locked down since only one possible answer can complete the equation.
Voltage is supplied at a constant(ish) value from your charger block. And your headphones are constructed to have a specific natural resistance. So, generally, if you connect the headphones to a circuit of the correct voltage, the desired current *must* flow through it.
In real life this is, as you can probably guess, not so clean cut. For one, your device’s resistance is almost certainly not a fixed value. A battery has more resistance the fuller it is, so a charging battery can pull more current when it is low than it can when it is mostly filled up. Additionally, charging circuits may contain components that can change the device’s effective resistance on the fly, allowing the device to actively control how much current it draws.
This leaves one last detail: the amp rating on the charger. If the device pulls whatever current it wants, what does the current on the charger mean? It’s simply the max current that the charger is designed to handle. Exceed it, and the charger may heat up and melt, or even catch fire.
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