Please note that you have to type in the commands exactly as mentioned in my post. There is no tolerance for errors when working with the terminal (not even one single character).
I sense that you might not be familiar with the terminal application or with how files and directories can be accessed. What these commands above do is the following:
This changes into the directory Downloads inside your home folder (~). In Downloads, it changes into subdirectory 'qcad-3.4.6-linux-x86_64'. This is the directory (= 'folder') where QCAD was extracted. The slashes ('/') in that command are vital. They are used to separate directories.
This starts (executes, runs) the QCAD application binary.
You can do the same also in a file manager, for example Nautilus or whatever file manager you are usually using to manage your files and directories. All you have to do is open the directory 'Downloads' in your home directory, then open the directory 'qcad-3.4.6-linux-x86_64' and in there run the file 'qcad'.
If all this makes no sense whatsoever, it might be a good idea to ask someone for help who is familiar with how files and directories are organized on a computer operating system such as Linux.