How to create a plugin with backend API calls to populate composer while drafting?

[Arkshine]

@fabkosta

Question: Where can I find docs for DModal, DButton, and other such Discourse-specific objects?

Here:

Using the DModal API to render Modal windows (aka popups/dialogs) in Discourse

Discourse 3.1.0.beta6 ships with a brand new <DModal> component-based API. This supersedes the old controller-based API, which is now deprecated. If you have existing modals using the old APIs, check out the migration guide here. Rendering a Modal Modals are rendered by including the <DModal> component in a handlebars template. If you don’t already have a suitable template, check out Using Plugin Outlet Connectors from a Theme or Plugin. A simple modal would look someth…

Also worth noting:

Discourse toolkit to render forms

Basic Usage FormKit exposes a single component as its public API: <Form />. All other elements are yielded as contextual components, modifiers, or plain data. Every form is composed of one or multiple fields, representing the value, validation, and metadata of a control. Each field encapsulates a control, which is the form element the user interacts with to enter data, such as an input or select. The control type is specified via @type on the field. Other utilities, like submit or alert, are …

You can look at Discourse/plugins/theme components for examples.