Creating Symlink To a File

At terminal :

ln -s source_file symbolic_link

https://linuxize.com/post/how-to-create-symbolic-links-in-linux-using-the-ln-command/#creating-symlink-to-a-file

How to mount hard disk for backbox ubuntu to extend space on virtual machine(error No Space Left)

You can use GParted and benefit from the GUI they offer.
or you can follow the next steps using the CLI:
http://manpages.ubuntu.com/manpages/bionic/en/man8/mount.8.html