@whirly it's not a proper system it's just the commandline interface of it. if you installed linux on a separate hard drive for example it'd have a GUI and it'd have an app store and stuff. some things you'll still have to use the terminal though. you're just typing commands with arguments, the arguments are what the commands use to know what to do.
very simple command
apt install telegram
^ apt is the package manager
^ install is the subcommand
^ telegram is argument for what to install
and all commands basically follow the same thing. command, maybe subcommand, argument. there's also optional arguments that you can put anywhere by putting -- or - in front of them