For the grouped template parts, if the theme does not define an area (in theme.json) for a template part, it will be grouped in General (uncategorized).

For example TT1 Blocks theme defines the template part areas like this:

"templateParts": {
"header": {
"area": "header"
},
"footer": {
"area": "footer"
}
}

This seems to be missing from the theme developer documentation.