

Run apt update and see if it works. Meaning to look what sources it connects to, if it doesn’t fail.
If this fails, then ofc you have to create the file with the Trixie sources. There has to be a dummy file somewhere for you to copy.
I don’t really understand your post, you claim to have updated, but it’s a new usb install?
You use ventoy to boot into a live linux iso and then run gparted to resize, though there is a chance you have to do a non persisting install of gparted first.