Desktops don't always use NetworkManager's programs
Three things are definitely true on modern Linux and modern Linux
desktops. Pretty much everyone is using NetworkManager, NetworkManager
includes GUI frontends, and desktops have GUI interfaces for
controlling your active networks and configuring them. In a nice
regular universe, the desktop's GUI frontends would be using the
NetworkManager GUI frontends like
and would thus usually automatically support everything that
NetworkManager itself supports (since the NetworkManager developers
tend to update their GUI frontends when they add features like
support for WireGuard or for "metered" connections).
Unfortunately this isn't a nice regular universe, so several major
desktops do not use the NetworkManager programs for their GUI and
as a result can be missing support for NetworkManager features.
GNOME and Cinnamon definitely use their own code for both controlling
active network connections (what
nm-applet is used for) and for
configuring network connections (what
used for). Cinnamon's version doesn't support WireGuard VPNs or
setting connections to "metered" status, as I've found out over
time; I don't know about the state of GNOME's. I believe that KDE
has its own applet; I don't know if it uses the NetworkManager
(The GNOME and Cinnamon desktop shells both implement applets as
instead of as separate programs. However, Cinnamon can use
instead of its own thing if you do the right magic things. I don't
know about GNOME.)
Generally you can run
nm-connection-editor directly if you want
to (and remember what it's called); it appears to work when run
directly even in a GNOME Wayland session. Some desktop environments
(such as Cinnamon) may offer you a confusingly named additional
"applet" menu option that runs
nm-connection-editor instead of
the desktop's own connection editor (on my Cinnamon desktop it's
called "Network Connections", instead of the "Network Settings"
that invokes Cinnamon's own, limited version). Unfortunately as
far as I know there may be no way to run
nm-applet instead of
your desktop's less up to date version, and even if you can you
may lose other features in the overall desktop environment.
You might reasonably ask why this matters. One of the reasons it's mattered for me in the past is that it can be rather confusing to read some online documentation and then not find what it's talking about in my Cinnamon desktop environment, because the online writing is talking about the official NetworkManager way (and may be written by someone on a desktop environment that does use the NM programs). It also complicates having full support for things like metered connections and WireGuard links, because it's not enough for them to be present in the official NetworkManager programs; they also have to make their way into all of the desktop reimplementations.
PS: The situation with nm-applet can be especially confusing in Cinnamon
(at least); my desktop session actually has a
running, despite not using it. A test GNOME session doesn't behave
this way. Since my Cinnamon desktop environment has been around for a
while (it looks like since 2013, since
even my laptop environment has been long-lived),
nm-applet may be inherited from old days when Cinnamon
actually used it.