Why have all my files from my old Macbook Pro synched to my desktop only

I just bought a new macbook pro and began to set it all up.

I was asked to update to the latest macOS Ventura, which I did.

When it was all installed and restarted, literally ALL my files (from various folders / downloads etc) from my old Macbook Pro synched and appeared on my desktop. Now my desktop is filled with hundreds of files and documents and appear to be linked to my old macbook.

Why did this happen?

If I delete a document from the desktop's new laptop then it deletes it from the relevant folder from the old laptop.

Is there a way of sorting these documents back to their rightful locations without deleting them or manually moving hundreds of files one by one which will be extremely time-consuming?

Will these files always be linked to my old laptop?

This has never happened previously when I have bought new Macs.