C# Use Microsoft.Office.Interop.Excel Create a Application and Manually opening an excel document generates two excel processes

Question

I am using vsto to create a word addin. During the process, I want to create an excel process through Microsoft.Office.Interop.Excel. However, when I manually open an excel file, I find that two processes are generated in the background. Why is this? How can I avoid this as it prevents me from using excel formulas across files