Here are some lesser known facts about wired headphones that I wish I had known earlier:

- there are TRS (3 rings) and TRRS (4 rings) connectors

- TRRS has integrated mic, also ofen implements playback control via buttons

- Apple headphones volume controls won't work on non-apple devices, because they implement a proprietary protocol based in an ultrasonic chirp authentication chip[1]

- 1more headphones seem to have implemented this auth protocol, but it does not work reliably

- Headphone remotes on apple devices have tap codes (in my opinion a huge benefit), that can be used to

  play/pause (.)
  next chapter / track (..)
  prev(...)
  fast forward (._)
  rewind (.._)
  even navigate menus (_, then + or -)
  
  ---- legend ----
  .=click
  _=hold 
  +-=vol

- The same tap codes are also implemented on hardware buttons (e.g. iPod) - while fast forward and rewind only work for devices iPod Nano 6th or later (iPod Classic 2009 only has next and prev)

- Apples USB-C 3.5 Adapters work with Android and iPhones and all headphones, Android does only implement play/pause and volume

- headphones remotes are fundamentally broken on Android because Google reserves longpress (button hold) for voiceover

- there are also balanced (often 4.4mm) outputs for much higher quality, often used in high res daps from fiio or shanling, etc

1: https://tinymicros.com/wiki/Apple_iPod_Remote_Protocol