Where does the finger command takes the information about the write permissions of the tty's

I'm trying to create my own

command in C in Linux Mint. Studying the real

command I've understood that the asterisk that the commands put when using the short output format left the tty is when one of the user has the write permission disabled, the permission used to send and receive messages between tty's I assume, that can be enabled with the

mesg y

command. The point of my question is, where does

gets the information about the write permissions of the tty? I've tryied looking into the utmp file but I can't find something relevant :( Also, if I disable the write permission on the tty connected with the GUI, why does the

command put the asterisk on the pts/0 entry, instead near my tty? like

matteo      tty7     2:40  Apr 29 16:10 (:0)
matteo      *pts/0         Apr 29 17:20

thanks in advance and sorry if I wrote something incorrectly, it's my first time going this deep in Linux :)