If the plugin is used as standalone plugin, it can be dropped as plain file, or inside a folder into your plugins or MU-plugins folder. Then it can be activated from the admin UI "Plugin" list.
When you're running a WordPress Multisite/Network installation, it's best to simply use a muplugin, so you don't have to care about activating it on each network site. Simply add the following to your wp-config.php file:
define( 'WPMU_PLUGIN_DIR', WP_CONTENT_DIR.'/mu-plugins' ); define( 'WPMU_PLUGIN_URL', WP_CONTENT_URL.'/mu-plugins' );
The plugin will run with custom Plugin URLs/domains, custom plugin and mu-plugin folder locations.
https://example.com ├── wordpress-root │ ├── wp-admin │ │ └── ... │ ├── wp-content │ │ ├── plugins │ │ │ ├── wcm-dir │ │ │ │ └── dynamic-image-resize.php │ │ │ ├── ... │ │ │ └── dynamic-image-resize.php │ │ ├── mu-plugins │ │ │ ├── ... │ │ │ └── dynamic-image-resize.php │ │ └── themes │ ├── wp-includes │ │ └── ... │ │ ... │ └── wp-config.php ├── custom-plugins-dir │ ├── wcm-dir │ │ └── dynamic-image-resize.php │ └── dynamic-image-resize.php └── custom-mu-plugins-dir └── dynamic-image-resize.php https://plugins.dev └── plugins ├── wcm-dir │ └── dynamic-image-resize.php └── dynamic-image-resize.php https://mu-plugins.dev └── plugins └── dynamic-image-resize.php