No. This is not the answer to my question - at all. I already mentioned I don't want to 'add custom code' but to 'modify existing code' to improve accessibility.
I don't want to add custom code. I want to edit (improve) the existing code to improve accessibility.
For instance;
This metatag in the head is throwing an error in lighthouse.
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=yes"/>
I want to change the maximum-scale to 5.
I have found where to do this in //plugins/system/t4/themes/base/html/layouts/t4/index.php
But of course this is editing the core and not an override.
How do I make the override so that the customisation is not overwritten on future updates?
The layout builder does not give access to the <head> as far as I can see.