Simple trick

It still is if I'm correct but most operating systems have an option to spoof/randomize your MAC address

It is, but there are ways to spoof it so your device presents a different one when connecting to a network.

Not quite. I'm not really sure but I think the original idea actually was a fixed hardware address but I'm not sure if a lot of devices actually ever implemented it that way because it's simpler (and cheaper) to control it in software. In modern (especially mobile) devices it's actually a security requirement because with a fixed MAC address you could be tracked by other wifi devices.

With the right software, you can do anything!

As the others said that is normally the case but nowadays most computers and mobiles have an option that randomize the MAC addresses on each connection.

These MAC addresses are known as locally-administered address. They look like this:

 undefined

        x2:xx:xx:xx:xx:xx
    x6:xx:xx:xx:xx:xx
    xA:xx:xx:xx:xx:xx
    xE:xx:xx:xx:xx:xx


  

And rarely like this:

 undefined

        x3‑xx‑xx‑xx‑xx‑xx
    x7‑xx‑xx‑xx‑xx‑xx
    xB‑xx‑xx‑xx‑xx‑xx
    xF‑xx‑xx‑xx‑xx‑xx