there were 2 scripts that semeed related to that: 10_linux_proxy and 35_linux_proxy.
There is a folder called proxified scripts, and inside it there are two files: linux and os-prober
Here is the text in the linux file: https://textdoc.co/V3atnuEvcG4QlPUp
I'm not sure what to do with it
it's not working, also I don't know why when I went in that line I have a \ now, that wasn't there before apparently, before $menuentry:
echo "submenu '$(gettext_printf "Advanced options for %s" "${OS}" | grub_quote)' \$menuentry_id_option 'gnulinux-advanced-$boot_device_id' {"
(I've tried adding it also in front of ${CLASS}, leaving it only were it is, and removing it from both)