Apple - how to change "§" (section symbol) key to write "`" (back quote)

An easier way with Karabiner is to just add a new Simple Modification (the first tab in Karabiner):

From key: non_us_backslash
To key: grave_accent_and_tilde (`)

Here's a screenshot of that:

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Why is that called non_us_backslash and not §?

The "non us backslash" is probably any key directly below the esc key (or the touchbar) (source: this issue).

On the English International, UK English, Dutch, Polish etc. keyboards, that's the key that produces the section symbol §.

You can use Karabiner to remap keys on your keyboard.

<?xml version="1.0"?>
<root>
  <item>
    <name>Section to Back quote</name>
    <identifier>sectiontobackquote</identifier>
    <autogen>
      __KeyToKey__
      KeyCode::UK_SECTION | ModifierFlag::NONE,
      KeyCode::BACKQUOTE
    </autogen>
  </item>
</root>