Content entities include node and any other custom entities. There are few ways to add comment to entities.
Option 1 - Add a comment field via the admin interface
Use the admin interface to create a comment field for you entity. Whether it is a node or a custom entity it is the same process. Use the steps here to make it happen. Follow the instruction here to add an comment to an entity using the UI
- Easier to implement than from code.
- You can use the feature module to store configurations as code and manage configuration updates. This the easiest to manage if it is a custom module that you own.
- This field is not part of the base fields, so not in the base entity table. Not a big issue though.
- This is a custom module that you or your team owns. The module is not public.
- You use the feature module for manage and update field configurations.
Option 2 - Programmatically add this field to base field definition of your entity
A second option is to add comments as a base field of the target entity type. The example here provide a list of steps.
- Your field is part of the entity base fields
- Require programming.
- You need to also write forward compatibility. Required if your module is already installed and an update is required.
- A contrib module that require update and forward compatibility.
- You prefer to have comments as a base field.