How does the BitConverter reconstruct bytes to a short

So I'm currently trying to learn more about endianness and how bytes are converted to shorts, ints etc.

And I figured I'd start simple, I'd start by converting a short ("30000") into two bytes and storing it in a MemoryStream

private static void WriteShort(short constValue)
{
    _stream.WriteByte((byte)(constValue & 255));
    _stream.WriteByte((byte)(constValue >> 8));
}

and if I understand things correctly, I start by doing some bitmasking using the bitwise AND operator &.

0000 0000 1111 1111 &
0111 0101 0011 0000

Which would result in a byte that looks like this 0011 0000 and I would write that to the MemoryStream. So now the MemoryStream contains a byte array which looks like this [48]

I then write another byte, based on the same value 30000 but I bitshift the bytes to the right by 8 in order to get the left hand most 8 bits 0111 0101 (117) and write that as a byte to the MemoryStream. So now the byte array looks like this [48, 117]

that part seems pretty clear to me, it's the reconstructing of the short which gets me slightly confused.

How come I need to do the same bitshifting when reconstructing the short? I think that question is somewhat related to my other question which is, how does the '+' operator convert '48' and '117' to 30000?

And how does the BitConverter.ToInt16(_stream.ToArray()); know what bytes to shift etc to output the correct value?

private static short ReadShort()
{
    _stream.Position = 0;
    return (short)((_stream.ReadByte() & 255) +
                  (_stream.ReadByte() << 8));
}

The entire program

internal class Program
{
    private static MemoryStream _stream;
    static void Main(string[] args)
    {
        Console.WriteLine(117 << 8);

        _stream = new MemoryStream();
        short constValue = 30000;
        WriteShort(constValue);
        var v = ReadShort();

        /* True */
        Console.WriteLine($"Is Little Endian: {BitConverter.IsLittleEndian}");
    }

    private static void WriteShort(short constValue)
    {
        _stream.WriteByte((byte)(constValue & 255));
        _stream.WriteByte((byte)(constValue >> 8));
    }

    private static short ReadShort()
    {
        _stream.Position = 0;
        return (short)((_stream.ReadByte() & 255) +
                      (_stream.ReadByte() << 8));
    }
}