I have few questions about ubuntu software ...

The default software is determined by the metapackage "ubuntu-desktop" I assume you mean vanilla Ubuntu. If you click on the metapackage you like (Kubuntu has different defaults) then you will see what is pulled in
https://packages.ubuntu.com/search?keywords=ubuntu-desktop&searchon=names&suite=hirsute&section=all

The other software you can install is also searchable on the same site but you can search the Software Centre for applications to install and they will be pulled down from the repositories and installed as required

hi ... thank for ur reply but I still do not get it...
Is alsa-base a software in ubuntu ? and is it already installed by default or that software need to be installed ?

It is a package in Ubuntu and is default installed
https://packages.ubuntu.com/search?keywords=alsa-base&searchon=names&suite=hirsute&section=all

Its not a software that you run as a user, like a web browser or text editor but it is a package which puts the below files on your system
https://packages.ubuntu.com/hirsute/all/alsa-base/filelist

If you want to see all possible software that can be installed from the Ubuntu software repository, then you best look at https://packages.ubuntu.com/focal/ and all its sub-pages (in total more than 90,000 software packages). Additionally software that is packaged as snap package (snapcraft.io), appimage or flatpack can also be used on Ubuntu.

If you ask about the list of packages that come with an initial install of Ubuntu:
see https://packages.ubuntu.com/focal/ubuntu-desktop
All packages in that list are installed with a standard installation (and the packages that they depend on).
alsa-base is among them.

There are variants of Ubuntu available, e.g. Kubuntu (based on KDE), they come with slightly different lists of packages that are installed from the beginning, e.g. https://packages.ubuntu.com/focal/kubuntu-desktop