How to add some HTML <meta> tags to a page via a layout.xml file?

The real examples from the core

1. Magento_Theme/view/frontend/layout/default_head_blocks.xml

2. app/design/frontend/Magento/blank/Magento_Theme/layout/default_head_blocks.xml

3. app/design/frontend/Magento/luma/Magento_Theme/layout/default_head_blocks.xml

4. app/code/Magento/Backend/view/adminhtml/layout/default.xml

How is it implemented?

How is Magento\Framework\View\Page\Config\Reader\Head::setMetadata() implemented and used?

See also: