How does it know what to say back?
You give it both halves: the message you received and a line about how you want to answer. The incoming message sets the register, a joke gets a joke back, a serious message gets a serious answer, and your instruction sets the content. Without the second half you get something bland and generic, which is the most common reason people are disappointed by reply generators.
Can it read the subtext?
Often, yes, and the app has separate tools for exactly that. Decode a Message tells you what someone is probably getting at. Is This Flirting? and Vibe Check do what they say. Rate My Message scores a draft before you send it. If you are unsure what the message means, decode it first and then reply.
Does it sound like a robot?
Only if you let it. Ask for “short, lowercase, how I actually text” and you get that. The default is a notch more polished than most people text, so the single most useful instruction is usually “make it more casual”.
Where it works
Through the TextAI keyboard, which means inside iMessage, WhatsApp, Instagram DMs, Slack, Hinge, anywhere you type. You never copy text into a separate app and back.