Desktop state is neatly organized using freedesktop standards. Since components are so modular, it's trivial to replace or modify any of them using any client program you wish.
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This is peak "Year of the Linux Desktop" behavior
S76 devs have stated they're not going to expose all the possible configurations you can make to COSMIC via the settings app (taking a more GNOME approach rather than a KDE one) but that third-party developers can create COSMIC-tweaks esque applications to modify a part of a desktop beyond the exposed GUI settings. Applets will work in the same way with a handful of 1st party applets that can be accompanied by community ones.