I have an OPENQASM program that performs entanglement swapping. It has five qubits: the data qubit and four link qubits. It works, but I want to see the details of the Bell measurement transformation. So what I am looking for is a reference or technique that will help me construct the 32 by 32 matrix from the four-by-for matrix for the Bell measurement basis. I understand that this is written as a direct product or tensor product, but I don't know what that means in terms of matrix operations.
Something like $B \otimes I$? Again, what does this mean?
Of course, the general problem is, given an $n$-qubit unitary transformation, how to construct the $m$-qubit matrix where $m>n$?