I'm wondering where I can find the file manager's preferences on Ubuntu 19.10.
I tried to open the file manager and to click on the arrow near File at top left; I found only the options: Home, New Folder, Show Details, Esc.
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In previous Gnome Shell versions, Preferences was located in the application menu, i.e., the drop down menu you get when clicking the application name in the top bar. That menuhas been deprecated. Now, application specific options have been moved within the app itself (where they belong, in my opinion). What remains in the top bar menu are window management options.
In Files, the button with tree vertical dots (or three lines as a hamburger in other themes) pulls up the application menu. This is where you now will find Preferences. It is the button immediately to the left of the windows control buttons (max, restore, close) if you have these to the right.
Addon: On my 19.10 install (vanilla), there is no hamburger icon in the Files window. Selecting Edit->Preferences will do the trick.
...and if that does not work (title/menu bars have been known to intermittently disappear on all of ubuntu's window managers before) use Alt-E to open the Edit menu.