Excel has the built in MMULT function, but I’m not aware of any built-in support for eigenvalues or eigenvectors. Many people have written such functions though.

That said, I would be surprised if Excel spreadsheets were implemented as matrices. Since you can update one cell and have it automatically update any computation that uses that cell, I would expect spreadsheets to be implemented with some sort of dependency graph so it’s easy to traverse and update the values that need to be changed. (This could be implemented as an adjacency matrix, but I haven’t seen that representation used before for programming language dataflow analysis.)