What's the difference between SMS and RCS chat messages?

So, I’ve been noticing something kind of funny while I'm sending out text messages to people in my contacts, and right there at the top before you even start typing, the header changes depending on who I'm messaging. For some folks, it shows up like this:
Text message (exchanging messages with unknown) (SMS / MMS)
But then, when I switch over to other people in my list, it says something totally different, like:
Chat message (chatting with unknown)
I was sitting here wondering, why does it label it as a "Text message" for some people but call it a "Chat message" for others?
What's actually going on behind the scenes with that distinction?

If you're just using the standard Google app, a Text Message basically means you're sending a regular old SMS that hits your plan, whereas a Chat message means you're mostly just sending stuff over the internet.

But I mean, why does he get to decide on his own whether he's sending someone a standard text or switching things up to a
chat?
What kind of logic is he even using to make that call?

If the recipient has enabled chat within their settings, then that's primarily how the message will go through.

Hey, do you happen to know where I can actually check if my chat feature is turned on or if it's currently disabled?
Galaxy A04s - Android 12

https://support.google.com/messages/.../7189714?hl=en