Ubuntu packages are classified into four components based on two properties: a “fully supported” core vs other packages provided “as is”; free vs proprietary software. “Main” is fully supported and free, “restricted” is fully supported and proprietary, “universe” is as is and free, and “multiverse” is as is and proprietary. By default, Ubuntu only enables packages from the “main” component. If you tick this checkbox, the “restricted” component is enabled as well. You can enable “universe” and “multiverse” after the installation. You're asked about “restricted” during the installation because your hardware might require proprietary drivers without which the installation wouldn't work.
Here are a few ways to find the list of packages in a component.
On the Ubuntu website, there's a list of packages per release and section, e.g. 20.04 administration utilities. The component is in brackets if it isn't “main”, so you can search for e.g.
You can also read the full package list (e.g. 20.04), again with the component in brackets if it isn't “main”. There's also a package list with more information including package descriptions (that's actually the list that the package manager uses to know what packages are available, their dependencies, etc.), and this one is sorted by component, e.g. 20.04 restricted for 64-bit PC.
Not all of these packages are installed by default, of course. I can't find a convenient list of packages installed by default. The manifest (e.g. for 20.04 desktop) lists much more than what's installed by default, but doesn't appear to include restricted software. I think what ends up being installed depends on your hardware anyway.
On your own Ubuntu machine, if you've enabled a component, you can read the package list at e.g.
/var/lib/apt/lists/archive.ubuntu.com_ubuntu_dists_focal_restricted_binary-amd64_Packages (adapt for your download mirror site, version, component and processor architecture). With aptitude, you can list installed packages from a component with e.g.
aptitude search '~srestricted ~i'
To list available packages and not just installed ones, remove