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:


Why is that called non_us_backslash and not §?
The &quot;non us backslash&quot; 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 §.