how did my macbook pro desktop show up on my macbook air desktop?

I have an ancient macbook pro (retina 2012) running macOS 10. On my wife’s 2019 macbook air (running OS 13) I created a second user profile for myself. I did this a few days ago and have used it a handful of times since. Today, as I was updating a couple apps on the macbook air, I noticed that my macbook pro desktop was now on the air’s desktop. As in all the folders and documents from the mbp were now sitting on the mba desktop where it had just been a wide open screen seconds earlier. This is in no way a bad thing. In fact I like it. However, my question is - did I unwittingly do something to have the mbp folders and docs show up directly on the mba desktop? Obviously I knew the mbp desktop would be available on the mba through icloud drive, but I thought I would have to go grab them when needed. I wasn’t expecting them to port themselves over. And, will they pop up on my wife’s desktop as well since we share an apple id?

I’ve accessed my mbp desktop items tons of times on my ipads and phone. But this is actually the first time I’ve connected with another laptop.