A way to add data to the custom templates placed in the /templates folder:
{
"$schema": "https://schemas.wp.org/trunk/theme.json",
"version": 2,
"customTemplates": [
{
"name": filename",
"title": "Template Name",
"postTypes": [
"page"
]
}
],
"settings": ...
}
Source: https://developer.wordpress.org/block-editor/how-to-guides/themes/global-settings-and-styles/#customtemplates