Just as an addendum to your answer. In the command writing to mullvad.list the | sudo tee /etc/apt/sources.list.d/mullvad.list is using two helpful linux utilities to modify the command. The first is the | which is called a pipe and connects the text output of one program to the text input of another. The pipe is connecting the output of echo which simply prints a string, in this case composed of the outputs of several other commands to the program tee. Tee which is given admin privileges by the sudo takes an input stream and splits it between two files. In this case those are mullvad.list and since no other was provided stdout the output pipeline of the terminal running the command.

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In the interest of further completeness. Another utility used in those commands is the command substitution operator of sh. So when the terminal is interpretting text $(some command) gets substituted out for the text output by the command in the parentheses. It is another common way of connecting commands on the shell to allow for more flexible and powerful commands.