C# - Use variable like type / Generic method

I am trying to refactor below two methods

method 1]

public void DoSomething(Bus a, string str)
{
   var temp = new List<Bus>();
   ...
}

method 2]

public void DoSomething(Car a, string str)
{
   var temp = new List<Car>();
   ...
}

Below doesn't work, and gives 'a is a variable, but used like a type' error. In addition to that, I can't even imagine how to call this method / what to put in the first parameter.

public void DoSomething<T>(T a, string str)
{
   var temp = new List<a>();
   ...
}

DoSomething<Bus>(Bus, "str");
DoSomething<Car>(Car, "str");

1. Other posts suggest to use MakeGenericMethod. Is this the only way? Use variable as Type
2. If I want those methods to return List<T> (Car / Bus) instead of void, how can I use #1 solution with Generics?

Thank you very much in advance!