To get a list of packages installed locally do this in your terminal:
1apt list --installed
or
1dpkg -l
or
1 dpkg --get-selections | grep -v deinstall
(The -v tag “inverts” grep to return non-matching lines)
To get a list of a specific package installed:
1apt list --installed | grep postgresql
or
1dpkg --get-selections | grep postgresql
To save that list to a text file called packages on your desktop do this in your terminal:
1apt list --installed | grep postgresql > ~/Desktop/packages
or
1dpkg --get-selections | grep -v deinstall > ~/Desktop/packages
To find information about a package you are interested in installing:
1apt info postgresql