Can't copy documents/files to internal HD

I've just upgraded to Catalina on my 2015 Mac, and 'migrated' my files from my older mac (2009) to this one, and realized not all the files moved over, including my photos. So I copied/exported the files I wanted to an external SSD, figured that's the easiest way to transferred the files. But I can't seem to 'copy' any files to my internal HD, it would only copy onto the desktop, not where I want them. I was able to install new applications which shows up in the Application folder on the HD, but apparently have no 'permission' to copy files to the HD. I tried to change the sharing/permission status, but it won't let me even though I am the admin, even if I sign in as the 'root' system administrator. I came across some articles about how filing system is now different in Catalina, can anyone help and explain what I need to do? Thanks