There is no automatic integration on Android phones between SMS and another network the way iMessage is automatic.
Exactly, even though it is the default messaging app on Android and receives their SMS texts for them, most users don't realise they are using it or that when they "send a text" to another Android user it is actually sending the message over data.
Anyway my understanding is that a text is a SMS text, and a whatsapp is a whatsapp message. I know that Google Messages hides itself by sending SMS texts when the number isn't known to Messages, and I guess Whatsapp does this too. I also live in the UK and occupy a space within multiple different communities which insist on different messaging apps. May be I just don't the option to be so vague.
WhatsApp is the default