Why do some mailing list emails have an "unsubscribe" option & some don't?

I've just started to use the "unsubscribe" button for emails that I always just delete immediately. Things I may have subscribed to ages ago for one reason or another.

What I find weird is that some have the "unsubscribe" button & some don't, yet I'm sure they're all from companies where I've opted in to receive emails.

Is there some particular difference between these emails that dictates how I see them?

Is there some particular factor - how they got my email address for example - that makes the difference?

I know that I can scroll to the bottom & follow their own unsubscribe link & none of these emails are a problem so I don't need to do any blocking. I just can't figure out why some have the "unsubscribe" option & some don't.

And why isn't Apple Mail listed as a topic that I can choose?