Each plugin acts in its own iframe. The editor will connect the index.html file, specified in the config.json plugin configuration file. The index.html file is the plugin entry point, connecting the plugin.js file - the base file needed for work with plugins in the editors.
The <head>...</head> section contains the links to all the scripts and stylesheets necessary for the plugin correct work (both local and remote, if the plugin uses some). It also includes the link to the plugins.js base file needed for correct work with the editors and containing the base plugin method work.
If the plugin is visual and you want its buttons and elements to look like those in the editor, then you can connect the plugins-ui.js and plugins.css files. More information about ONLYOFFICE styles and elements can be found here.
The body can contains the <div>...</div> tags with the placeholders where the plugin components will be inserted. The behavior of these plugin components is described in the file with plugin code itself.