Same apps as administrator as standard user?

An admin user and a standard user are exactly the same. There is no difference in the account. Accounts marked as admin get added to the sudoers file which enables them to elevate their privileges. Otherwise, there is absolutely no difference.

You will generally have to authenticate to install apps into the Applications folder, but you can do that from the standard user account. It will just ask for admin user credentials when you try.

As for files shared between the two, that is not simple, as I stated above.

It certainly is much more difficult with things like music, photos, etc. Those programs do not expect that their files are shared amongst multiple users. You could store all of your music somewhere in /Users/Shared, then set up Music to not copy the files into its library, but I imagine that will create some problems. I think you can set up Photos similarly, but I don't know how the referencing by two accounts will play out.

What is the point of the separate user accounts. As I stated, there really is no difference. Running as a standard user mostly protects you from yourself. With Catalina and Big Sur, nothing, including root, can modify the system. It is mounted read-only. Even if something tricked you into authenticating and elevating its privileges, it could not do anything to the OS. root, as a vector for attack, is virtually useless in macOS.

I should be fine using 1 administrator account then :)

The only way you can get Malware is to install it. You can install it while logged into a Standard user account by authenticating with the admin username and password. So, operating as a Standard user when you know the admin username and password does absolutely nothing for you.

You just have to not install malware.

Effective Defenses against malware.

I would like to have the same Apps, files and configurations for both accounts , without having to install everything twice .

I tried it before with making a new admin. account after everything was installed on the first one , but I had no same apps and files .

How did you install the apps? By default, they should be installed in /Applications and every user has access to them.

The only way to prevent other users from accessing an app would be to install the app into the user's home folder.

Also, users are independent with respect to files stored within their home directory. You can store files in /Users/Shared so all can access them, but the default permissions makes the person who created the file the owner and doesn't allow others to edit the files. You can set up Access Control List entries which will allow members of a group to all edit files created by others in the group.

As said, as long as it is in the apps directory, every user has access. A good example is Photos. All users have access to the same program, but each user has a library in their own Pictures folder.

That is the standard setup. It should work fine. Note that sharing data between users is involved, though not difficult.