Plugins are loaded through the plugin_handler. It looks for directories in the Plugins folder and loads the loader.php file for each of those plugins.

Loader.php

The loader file below is a basic example of a plugin. It registers itself to the reactor global.

namespace Triekster\Reactor;

/** Plugin File
 * Name: Example Plugin
 * Copyright (c) 2020 Patriek Jeuriens
 */

global $reactor, $factory, $routing;

$plugin = array(
    "name" => "Example Plugin",
    "description" => __("Adds an empty plugin."),
    "plugin" => basename(dirname(__FILE__)),
    "author" => "Patriek Jeuriens",
    "license" => __("Commercial License"),
    "version" => 10222020, "enabled" => true
);

$reactor->plugins[$plugin["plugin"]] = $plugin;

if ($plugin['enabled'] == true) {
}

Now your plugin is loaded and any code between the plugin['enabled'] statement will be executed.