Sending text messages from iPad or MacBook Air to an android or google phone

Of course, I can send text messages from my iPhone to any other type of mobile phone. But it seems that sometimes I can also send text messages from my iPad (which only uses WiFi) or MacBook Air to an android or google phone. And text messages from someone who has an android or google phone often show up in the Messages app on my iPad and MacBook Air. Yet, other times, it doesn't seem to work. Can someone please clarify this? Thank you.