I have laptop with a Portuguese (PT) keyboard layout and recently I've bought a Keychron K2 which has an US layout. I find it annoying to have to change the keyboard layout to whatever keyboard I'm currently using so I started to try ways to set each keyboard it's proper layout automatically.

After a lot of struggles trying to understand X11 config files I think I finally got it, and it all comes down to the creation of a 01-keyboard.conf file at /etc/X11/xorg.conf.d/ folder, with the following content:

Section "InputClass"
        Identifier "Keychron_K2"
        MatchVendor "Keychron"
        MatchProduct "Keychron K2"
        MatchIsKeyboard "on"
        Option "XkbLayout" "us"
        Option "XkbVariant" ",intl"
        Option "XkbOptions" "compose:ralt"
EndSection

With this configuration file X11, the thing that handles Monitors, Keyboards, Pointing and Touch Devices, is able to proper configure the keyboard layout whenever I have it plugged in.

What is happening here?

Whenever you plug a device to a Linux machine, its driver gets loaded by the kernel. You can check the status of that loading processing, and its messages, by running the dmesg command.

Running it on my machine gave me the following results:

[...] usb 3-1: new full-speed USB device number 6 using xhci_hcd
[...] usb 3-1: New USB device found, idVendor=05ac, idProduct=024f, bcdDevice= 1.12
[...] usb 3-1: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[...] usb 3-1: Product: Keychron K2
[...] usb 3-1: Manufacturer: Keychron
[...] input: Keychron Keychron K2 as /devices/.../usb3/.../input/input33
[...] apple 0003:05AC:024F.000C: input,hidraw0: USB HID v1.11 Keyboard [Keychron Keychron K2] on usb-0000:04:00.3-1/input0
[...] apple 0003:05AC:024F.000D: Fn key not found (Apple Wireless Keyboard clone?), disabling Fn key handling
[...] input: Keychron Keychron K2 as /devices/.../usb3/.../input/input34
[...] apple 0003:05AC:024F.000D: input,hiddev98,hidraw5: USB HID v1.11 Keyboard [Keychron Keychron K2] on usb-0000:04:00.3-1/input1

As you can see, dmesg shows details about the device and its manufacturer. From here, it is just a matter of leveraging that information to specify which settings we want for your keyboard.
And for that you need to create a X.org configuration file.

You can check the X.org conf documentation to better understand how it works but, for simplicity, think of it as a way to select a device, based on its characteristics and features, to be able to a specify configuration or command for that set of devices.

The are a lot of possible settings for those Section file, but for the keyboard layout, I think the following are enough:

1. Section "InputClass" The InputClass indicates that we are about to specify configurations for a input device.
2. Identifier This entry is required and it should be an unique name. Kind of like an Id for this InputClass
3. MatchVendor This entry tries to match the device vendors name with the specified name.
4. MatchProduct This entry tries to match the device product name with the specified name.
5. MatchIsKeyboard This is entry matches only if the device type is a keyboard.
6. Option "<OptionName>" "<OptionValue>" These entries basically defines options for the xkb command, that should be executed for all of the matched devices. You can specify any option or layout that xkb accepts.

After creating that file you are pretty much done. Just logout or reboot you computer and it should assign the specified layout to each device you've identified.

References

Xorg Conf
Input Device Configuration
Input Class Documentation
Compose key
Xkb Configuration