A WhatsApp newsletter or message is nothing more than a WhatsApp campaign where the same message is sent to a large contact list. Previously, this type of message was widely used in emails and is still used but to a lesser extent. However, this has changed and this type of campaign has had great relevance in mass messaging apps. This is because through these apps users communicate directly and in real time with companies, which facilitates and streamlines transactions and sales.
One thing you should keep in mind is that WhatsApp newsletters are not the same as broadcast messages or group messages. While in groups the messages are seen by all the members of the group and they can communicate with each other, in a newsletter there can only be communication between the sender and the receiver of the message.
Something very important is that newsletters in WhatsApp are sent with WhatsApp Business API so you must have an API provider like Callbell to do this.