An orthophoto with no georeferencing data cannot be managed properly inside a GIS software.
It has lost the chance of being explored in the software.
It could be for many reasosns.
A very frequent one could be a conversion of the file from tif to jpg (in order to lower its size) and, after too many emails and file transfers, it is no longer possible to go back to the original source.
Two orthophotos of close areas, even withi pictures shot at different times, each one different from the other and both without goereferencing data, can be combined thanks to a photo editing software to create one single file and image.
There must be a minimal overlapping zone between the two pictures.
Once stitched together, if you know the coordinates of at least four points, visible in the merged image, you can reference the result back in QGIS again.
Photoshop is for sure the most famous and used photo editing software.
It has been around for a long time and it’s really powerful.
To combine these two orthophotos I used the “Photomerge” option.
The software uploads the files, compares them and merges the images (but keeping them always separated).
Where the two images stitch together it automatically creates masks that allow great results in the merged image, keeping the original files unchanged.
Over the years this tool has improved a lot!
And it is an excellent help to support these technical data.