Example on how to (low level) hard remap a keyboard key on linux

HardRemapLinuxKeyboardKey.txt

$sudo evtest #the keyboard was input 5 here, I wanted to make KeyPad0 work as LeftAlt key

Select the device event number [0-22]: 5
Input driver version is 1.0.1
Input device ID: bus 0x3 vendor 0x1a2c product 0x3d43 version 0x110
Input device name: "SEMICO USB Keyboard"

Event: time 1599799246.967341, -------------- SYN_REPORT ------------
Event: time 1599799247.011339, type 1 (EV_KEY), code 56 (KEY_LEFTALT), value 2
Event: time 1599799247.011339, -------------- SYN_REPORT ------------
Event: time 1599799247.011339, type 4 (EV_MSC), code 4 (MSC_SCAN), value 700e2
Event: time 1599799247.011339, type 1 (EV_KEY), code 56 (KEY_LEFTALT), value 0

Event: time 1599797919.619347, -------------- SYN_REPORT ------------
Event: time 1599797919.659355, type 1 (EV_KEY), code 82 (KEY_KP0), value 2
Event: time 1599797919.659355, -------------- SYN_REPORT ------------
Event: time 1599797919.659355, type 4 (EV_MSC), code 4 (MSC_SCAN), value 70062
Event: time 1599797919.659355, type 1 (EV_KEY), code 82 (KEY_KP0), value 0

$sudo input-kbd 5
0x70062 =  82  # ???

$#the b*v*p* is based on "input device id" line from `evtest`
$#the =leftalt is just KEY_LEFTALT w/o KEY_ and in lowercase
$cat >>/etc/udev/hwdb.d/98-custom-keyboard.hwdb
evdev:input:b0003v1A2Cp3D43*
 KEYBOARD_KEY_70062=leftalt

$sudo udevadm hwdb --update
$ls -l /etc/udev/hwdb.bin

$sudo udevadm trigger --sysname-match="event*"

$#to restore use
evdev:input:b0003v1A2Cp3D43*
 KEYBOARD_KEY_70062=kp0