If you want to install deb packages in the command line, you can use either the apt command or the dpkg command. The apt command actually uses the dpkg command underneath it, but apt is more popular and easier to use.
If you want to use the apt command for deb files, use it like this:
$ sudo apt install path_to_deb_file
If you want to use the dpkg command for installing deb packages, here’s how to do it:
$ sudo dpkg -i path_to_deb_file
In both commands, you should replace path_to_deb_file with the path and name of the deb file you’ve downloaded.