How do you backup all your emails, by copying the 'Outlook' folder? (Outlook 2013)

[PC Advisor UK:] [...] All versions of Outlook use pst files, but the disk location has changed over the years. Right-click your email account on the left in Outlook and select Data File Properties. Click Advanced to see the filename. Only the pst file is essential, but copy the whole folder to back up Outlook’s settings and preferences too. Just copy it back to restore it, or click File, Open, Open Outlook Data File. Outlook can work with any pst file stored anywhere.

[Howtogeek.com:] Backing up your Outlook data file(s) is as easy as copying your .OST or .PST files over to another hard drive, cloud server, thumb drive, or some other storage medium. You remember how to locate your data files?

Open the Mail control panel and click the “Data Files” button. Click “Open File Location…” and you can then back up your data files.

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After following the instructions above, I arrive at:

C:\Users\[my username]\AppData\Local\Microsoft\Outlook

However, all Outlook Data Files here are .ost, and NOT .pst. In File > Open & Export > Open Outlook Data File, 'Open Outlook Data File' only works for .pst.
What is wrong here? Where's the folder with all the .pst files?
How can I backup all my Outlook email accounts simultaneously?

I'll assume you have Windows Explorer set to show extensions, since your question implies that. To find out where your .pst files are, in Outlook go to File > Account Settings > Account Setting and click on the Data Files tab. You should see a list of all your active Outlook data files and their full pathnames. Mine are in:

C:\Users\UserName\Documents\Outlook Files\

, and I have "Archive.pst", "Outlook.pst", and "Personal Folders.pst". I have this folder set as part of my routine backup procedure and use Norton 360 for that purpose. If you want you can schedule a task to copy this folder to an external drive each time you start your computer. I do this to copy my user profile pictures and wallpaper to my user account folders, which then get picked up as part of my weekly backups. You can create a .cmd file in the root of your system drive (typically C:\) that looks something like this:

@ECHO OFF
echo y | del C:\Users\UserName\Documents\Outlook Files\*.* /s
xcopy C:\Users\UserName\Documents\Outlook Files\*.* X:\Yourpath /y

and then add this file in Task Scheduler.

Why not just export to a .pst from the menu. Should be file > export > pst.