I thought maybe they got power from the phone but then I remembered old headphones plugged into a phone jack separate from the charging port so how come one type never needs to be charged and one does?
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Audio files, streaming services, etc, basically transfers audio in a digital format. The digital format is then used to reconstruct an analog electrical signal, which is then sent to a speaker to convert into vibrations (using magnets) that we can listen. That is the basic of how audio works.
Sending audio through a headphone jack means, your phone/laptop/media player first converts the digital audio file/stream into the analog electrical signal, and then amplifies this signal to a certain level, before sending the result out using the headphone jack to the earphones/headphones. How loud the sound coming out from your earphones/speakers, depends on the amplification of this electrical signal. For commercial earphones and headphones, the amplification needed is not that much, and so phones/media players are able to internally amplify the electrical signal, and send it to the headphones/earphones, at the cost of its own batteries. Old headphones uses slightly different magnets such that it needs more amplification for it to work properly, and so they have their own power supply to amplify the audio electrical signal before sending to the magnets to produce vibrations.
When it comes to speakers, where you’ll need it to be louder than earphones/headphones, the amplification required is larger, so more power is needed to amplify the electrical signal. Some speakers have built in amplifiers – basically if you see a speaker that is connected to a power source / uses battery, it has a built in amplifier. Even if the power source is a USB cable connected to your laptop/computer, that is still an in-built amplification – and others will require an external amplifier, but key point is, for an electrical signal to be heard, an amplification is needed.
When it comes to wireless technology, audio signal from the phone is sent through digital wireless communication, most commonly used being Bluetooth. So, the conversion from digital to analog electrical signal, happens on the wireless headphone itself. After that, again the electrical signal needs to be amplified to a certain level that it is audible, and that is why the wireless headphone needs its own batteries/power source. First, to power the digital to analog converter (DAC), and then to amplify this analog electrical signal.
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