man magick

https://www.imagemagick.org/script/command-line-processing.php
https://www.imagemagick.org/Magick++/Image++.html

Consider m = 2 monitors with the resolutions:

"resolutions": [
	{
	  "width": 3840,
	  "height": 2160
	},
	{
	  "width": 3840,
	  "height": 2160
	}
]

Consider 3 figures or images for each monitor:
fig01.avif  
fig02.jpg  
fig03.webp

Size occupied by each image:
3840 / 3 = 1280
2160 / 3 = 720



Example 1: different resolutions
monitor orientation: Side-by-side (horizontal)

magick -gravity Center \
\( fig01* -resize 3840x2160^ -extent 3840x2160 \) \
\( fig02* -resize 1920x1080^ -extent 1920x1080 \) \
\( fig03* -resize 3840x2160^ -extent 3840x2160 \) \
-gravity South +append wallpaper.jpg



Example 2: equal resolutions, horizontal + vertical
monitor orientation: Side-by-side (horizontal)

magick \
\( \
-gravity Center \
\( fig01* -resize 3840x720^ -extent 3840x720 \) \
\( fig02* -resize 3840x720^ -extent 3840x720 \) \
\( fig03* -resize 3840x720^ -extent 3840x720 \) \
-gravity South -append \
\) \
-resize 3840x2160^ -extent 3840x2160 \
\( \
-gravity Center \
\( fig01* -resize 1280x2160^ -extent 1280x2160 \) \
\( fig02* -resize 1280x2160^ -extent 1280x2160 \) \
\( fig03* -resize 1280x2160^ -extent 1280x2160 \) \
-gravity South +append \
\) \
-resize 3840x2160^ -extent 3840x2160 \
+append wallpaper.jpg



Example 3: equal resolutions, vertical + horizontal
monitor orientation: Side-by-side (horizontal)

magick \
\( \
-gravity Center \
\( fig01* -resize 1280x2160^ -extent 1280x2160 \) \
\( fig02* -resize 1280x2160^ -extent 1280x2160 \) \
\( fig03* -resize 1280x2160^ -extent 1280x2160 \) \
-gravity South +append \
\) \
-resize 3840x2160^ -extent 3840x2160 \
\( \
-gravity Center \
\( fig01* -resize 3840x720^ -extent 3840x720 \) \
\( fig02* -resize 3840x720^ -extent 3840x720 \) \
\( fig03* -resize 3840x720^ -extent 3840x720 \) \
-gravity South -append \
\) \
-resize 3840x2160^ -extent 3840x2160 \
+append wallpaper.jpg