When I have to show a series of experiment results plottings, I used to use Meitu (a phone photoshop app) to first airdrop all images to my iphone, concatenate all the images, and them airdrop back to my mac.
Not anymore now I got my hands on Imagemagick (https://imagemagick.org/).
ImageMagick® is a free, open-source software suite, used for editing and manipulating digital images. It can be used to create, edit, compose, or convert bitmap images, and supports a wide range of file formats, including JPEG, PNG, GIF, TIFF, and Ultra HDR.
Although when operated on PDF it loses the vector proporty but overall, being able to concatenate image by a single command operation in terminal is fascinating:
magick convert figure_1.pdf figure_2.pdf -append target_figure_name.pdf