Different package sources files for different systems package managers

[Bret Busby]

Hello.

On one of my computers running UbuntuMATE 21.04 (which is what I am
now running, on each of my computers), I performed a system update
today.

The results that I got, between using apt and the system Software
Updater, caused me to check the contents of /etc/apt/sources.list , as
(I believe) used by apt, and, the Software Updater -> Settings.

The result surprised me, as the result shows that the two package
managers use different package sources files.

How do I find what package sources file is used by the system Software
Updater, so that I can edit it, to ensure that the package managers
are using the same package sources?

-- 
Bret Busby
Armadale
West Australia
(UTC+0800)
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