Any guides on add custom fields to 'Post Settings"

Hello there, to clarify I mean these…

So i want to add, for instance, extra featured images, a quote(as well as excerpt) and featured video, obviously per post. I’ll need to add those in to the mysql/post but how then will i call them from handlebars? No idea how handlebars interacts with sql or where I can take a look.

Any guides out there or pointers?

Edit: I remember a few of the official ghost themes have this kind of functionality, I’ll just reverse it from them. If anyone has anything else it would be welcome.