From where do updates originate?

There are many approaches to software updates. Broadly, I characterize them as
the following:

• Local updates, often performed by a technician who carries the update on a
  portable medium such as a USB flash drive or an SD card and has to access each system individually
• Remote updates, where the update is initiated by the user or a technician locally, but it is downloaded from a remote server
• Over-the-air (OTA) updates, where the update is pushed and managed entirely remotely, without any need for local input

I will begin by describing several approaches to software updates, and then I will show an example using Mender (https://mender.io).