How can I specify which all products are available on COD?

You can manage which all products are available on Cash on delivery, to manage this, you need to install the Product based COD app. To install and configure follow the steps mentioned below:

You can install "Product based COD" app from Settings-> Advanced settings-> Apps, once the app is installed you would be able to select each product if it is available for COD or not through the product metafields.