I recently encountered a problem with the Nautilus file manager on Ubuntu 20.04, as it doesn't suite me while saving files, as described in my question here: Save File Dialogue Box in Nautilus, Ubuntu 20.04 Doesn't Show "Recent"?
As a result, I installed Nemo File Manager, and purged Nautilus. But, when I try to download a file from the web, it still shows the same "Nautilus-like" Save Dialogue Box, with which I find it very difficult to work...(even though running nautilus via terminal says Nautilus is not installed)
So, then my question is: Is the default Save Dialogue Box something engrained in the Linux / Ubuntu system which can't be changed? ...or Is it controlled by the file manager which is being used?
I guess that you use some application that uses GTK as GUI library. (You can check that by running xprop in a terminal and then clicking on the save dialog)
Now what controls the save dialogs?
It's the GTK library. So it has nothing to do with your file manager.
Look at this screenshot that I took in Linux Mint:
Nautilus isn't even installed but you get that nautilus like looking dialog. It's only because nautilus and the GTK dialog are designed to look similar.
What can you change about the look of those save dialogs?
You can only change the theming AFAIK.