Messages are sent as chats if the person you’re messaging has Message+, Rich Communication Services (RCS) and a data connection. Otherwise messages will be sent as text or picture messages. (This function isn’t supported on iOS.) You’ll always know which type of message will be sent (text, picture message or chat) before you send it.
You can see whether you’ve sent a text or a chat – most devices show them in different colours. Consult your device manual for specific advice.