Again, none of my OneDrive files in the Apple Files app show a cloud icon as your image shows. None of them open with airplane mode enabled, so they haven't been previously cached.
I disabled airplane mode, opened a file then re-enabled airplane mode. I was able to re-open the file again, suggesting it'd had been cached from the first launch. Still no cloud icons giving me any indication of the file cache sate.
This sort of cache isn't the same as the OneDrive option to selectively pick the files I want saved for offline viewing. What's to say I want everything I open cached on my phone? What happens when I clear the cache to clean up the phone, and then I want one of those cached files? Speaking of which, where do I go to clear that cache? :/
iOS has taken away from me the choice of what I want to "pin" or permanently save on the device. Again, some of the files are quite large and I don't want them downloading via data when I open them. With OneDrive, I know what's saved locally at a glance and I can choose to open that big file or not.