Does the WhatsApp cloud API support sending messages to Channels and groups?
I have seen external api providers like https://whapi.cloud (https://dev.to/whapicloud/how-to-send-a-post-to-whatsapp-channel-4na6) claim that they support sending messages to channels.
But I couldn't see any API related to channel in documentation at https://developers.facebook.com/docs/whatsapp/cloud-api/guides/send-messages/.
Is this kind of API available?
The official API from Meta does not have this feature. There, you can only send verified messages directly in person. And various independent solutions and libraries allow you to automate exactly your messenger, thus providing group/channel/community management functionality, including sending messages there. You need to weigh the risks yourself before using unofficial solutions. On the one hand you have access to automation of something that most likely won't be available in the Cloud API anytime soon. On the other hand, there is always the risk of having your number blocked, for example, for mass activity.