C# 11 (.NET 7) Introduced file scoped types:

namespace sampleNS
{
    file class Maths
    {
        public int Add(int x, int y) => (x + y + 2);
        public int Multiply(int x, int y) => (x * y * 2);
    }

    class Calculations
    {
        public int SomeCalculation(int x, int y)
        {
            Maths maths = new();
            return maths.Add(maths.Add(x, y), maths.Multiply(x, y));
        }
    }
}

They’re only accessible within the same file. They cannot be in different namespaces