USB is not that simple. Is it 5V, 9V, 12V, 20V?

• USB is not that simple. Is it 5V, 9V, 12V, 20V?
• How many amps are going through?
• Why is my plugged in phone actually discharging overnight?
• Should I plug my phone into this unknown USB plug with a data cable?
• (spoiler alert: no, it could be using data to try and hack your phone)

Simpler even: USB power monitoring