qmk-firmware/layouts/default/numpad_4x4/default_numpad_4x4/keymap.c

#include QMK_KEYBOARD_H

const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
    /*
     * ┌───┬───┬───┬───┐
     * │ 7 │ 8 │ 9 │   │
     * ├───┼───┼───┤ + │
     * │ 4 │ 5 │ 6 │   │
     * ├───┼───┼───┼───┤
     * │ 1 │ 2 │ 3 │   │
     * ├───┴───┼───┤Ent│
     * │   0   │ . │   │
     * └───────┴───┴───┘
     */
    [0] = LAYOUT_numpad_4x4(
        KC_P7,   KC_P8,   KC_P9,   KC_PPLS,
        KC_P4,   KC_P5,   KC_P6,
        KC_P1,   KC_P2,   KC_P3,   KC_PENT,
        KC_P0,            KC_PDOT
    )
};