Ubuntu – Ubuntu Software Center missing in 20.04

For some reason, both my Ubuntu Software Center (now simply called "Software") and the Snap Store weren't present on my Fresh Ubuntu 20.04 install. To install them, simply run the following two commands:

sudo apt install gnome-software  # "Software"
snap install snap-store          # Snap Store

Here's what their icons look like now when I press the Super key (Windows key on a PC keyboard, or Command key on a Mac running Linux), then search for them:

Ubuntu Software Center (now just called "Software"):

Ubuntu "Snap Store":

Now, to install a *.deb file using the GUI file manager, such as nautilus (default), or nemo (my preferred choice), simply right-click the *.deb file and go to --> Properties --> "Opens With" tab --> select "Software Install" (which is now available since you just installed gnome-software) --> and click "Set as default" --> Close.

Now you can just double-click the *.deb file in your GUI file manager to install it!

Note: you might also consider trying the gdebi package installer, as described here: Problem installing .deb in Software Install Ubuntu 20.04. Apparently, in some cases where the "Software Install" GUI option fails, and you would have to otherwise manually run sudo dpkg -i myfile.deb to install the *.deb file, the "GDebi Package Installer" GUI option works.

Other references:

1. This answer helped me identify the existence of gnome-software and the snap-store: Ubuntu Software doesn't open in Ubuntu 20.04