adding/changing xkb Symbols with altGr

I am running Ubuntu MATE 20.04 and read through much theory around xkb and I managed for example to change altGr-, from · to (a bigger, bolder) •:

$> testing keys... öäü…·
$> cp de de_frank  (yes, coming from a German keyboard)

    OLD   key <AB08>  { [ comma,  semicolon, periodcentered,  multiply  ] };
    NEW   key <AB08>  { [ comma,  semicolon,          U2022,  multiply  ] };

$> setxkbmap de_frank
$> testing key... öäü…• good :)

Problem / Question:

I want to replace altGr-shift-9 from ± to ? (U1F600)

Does work on trivial keys like shift-3, but I don't know where I can find the right place in the file. Line :115 (in my case, Ubuntu MATE 20.04) has NO effect regarding the 9 key with any modifieres whatsoever:

< key <AE09> { [ 9, parenright, bracketright, NoSymbol, fiveeighths, plusminus, bracketleft, NoSymbol ] };

> key <AE09> { [ 9, parenright, bracketright, NoSymbol, U1F600, U1F600, U1F600, NoSymbol ] };

Putting UF1600 on, say, shift-3 (usually the dollar sign) is not a problem. So the Problem is not with the use of a rather exotic unicode.

I am guessing, the settings for altGr-shift-9 come from elsewhere, the „partial“ in which my change sits gets never included, or latter overriden, I do not exactly see through the relations and priority of all this...

possibly useful:

$>  setxkbmap -print -verbose 10

Trying to load rules file /usr/share/X11/xkb/rules/evdev...
Success.
Applied rules from evdev:
rules:      evdev
model:      pc105
layout:     de_frank
Trying to build keymap using the following components:
keycodes:   evdev+aliases(qwerty)
types:      complete
compat:     complete
symbols:    pc+de_frank+inet(evdev)
geometry:   pc(pc105)
xkb_keymap {
    xkb_keycodes  { include "evdev+aliases(qwerty)" };
    xkb_types     { include "complete"  };
    xkb_compat    { include "complete"  };
    xkb_symbols   { include "pc+de_frank+inet(evdev)"   };
    xkb_geometry  { include "pc(pc105)" };
};