/usr/share/X11/xkb/symbols/eurosign is in xkb-data 2.10.1-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23  | // Most keyboards have the EuroSign engraved on the E key
partial
xkb_symbols "e" {
    key <AD03>	{ [  NoSymbol,   NoSymbol,   EuroSign,	NoSymbol ]	};
};
// Many Apple keyboards have the EuroSign engraved on the 2 key
partial
xkb_symbols "2" {
    key <AE02>	{ [  NoSymbol,   NoSymbol,   EuroSign,	NoSymbol ]	};
};
// Some keyboards have the EuroSign engraved on the 4 key
partial
xkb_symbols "4" {
    key <AE04>	{ [  NoSymbol,   NoSymbol,   EuroSign,	NoSymbol ]	};
};
// Many keyboards have the EuroSign engraved on the 5 key
partial
xkb_symbols "5" {
    key <AE05>	{ [  NoSymbol,   NoSymbol,   EuroSign,	NoSymbol ]	};
};
 |