How to use a C# class in many projects inside the same solution?

You create a class library project and in there you would want to add folders where you can name them according to what the classes would do so it is easier to maintain. Then you would add your classes in the corresponding folder and in the project that you want to use the class you can add a reference to the project and the class would be accessible as long as it was a public class.

Explanation

This is a bash script. The sooner you understand how to use dotnet CLI, the better.
CLI rarely changes. And it's like programming, not a recommendation to press a certain button.
In general, you don't know yet whether you will code in Rider or VS.
Eventually, you will be able to look at and debug the dotnet code itself, since it is open source.
Everything ends up in scripting. Otherwise, you will spend years wondering what that magic button does.
This script should be executed in bash console. It might be better to write it in powershell, but for things like this, I think bash is better.
cat with EOF is a very convenient command.
cat --help - explanation. or see https://man7.org/linux/man-pages/man1/cat.1.html
Try typing dotnet --help in any console to practice and understand.
On the left is the variable name, on the right is its value.
# starts comment.
Check it with the echo command, for example

sln=Abba
echo $sln
echo $sln.BoneyM

Scripts

    sln=mysln
    p1=$sln.app
    p2=$sln.lib1
    ver=net8.0
    dotnet new console  -n $p1 -f $ver
    dotnet new classlib -n $p2 -f $ver
    dotnet new  sln -n $sln
    dotnet sln add ./$p1/${p1}.csproj
    dotnet sln add ./$p2/${p2}.csproj
    cd $p1
    dotnet  add reference ../$p2/${p2}.csproj # add ref
    cd ..
    # create Baz.cs in library
    cat > ./$p2/Baz.cs <<EOF

    namespace $p2;
 
    public class Baz
    {
     public int Prop=1;
    }
    EOF
    # update program.cs
    cat > ./$p1/Program.cs <<EOF
    using $p2; 
    var boo= new Baz();
    Console.WriteLine(boo.Prop);
    EOF

    dotnet run  --project ./${p1}/${p1}.csproj  # test