Processing multiple files without distortion

What you want to do could be described as 'Fit the image' in a defined canvas, while keeping the aspect ratio. This results in adding white margins.  I can't test it now (I am not at home) but I think this is exactly what is offered in a script in the very affordable Elements+.

The script is:

https://elementsplus.net/help/en/fit-print.htm

It's called 'Fit Print'.

You have to define the script to adjust an individual image, then you have to follow the instructions to batch edit a folder of files with that script.

kbsmg  wrote .  But I wondered if there was a way to do a batch with the dimensions change but keeping the aspect ratio the same.

I have read your inquiry four times, and am unable to follow along towards your goal.

1500x1500 or 2500x2500 has aspect ratio = 1

1768x2160 does not have an aspect ratio=1 by my calculation.

Resizing with the caveat to retain the aspect ratio of the original will result in distortion. Think of a square net - as you pull on it in one axis the net will distort.

What you may be able to do is to crop the pictures to the desired specification, but that will result in loss of information and is not a batch process.