I have created an EPR pair. Let's suppose it's $(|00\rangle + |11\rangle)/\sqrt{2}$. We both of our halves and then we move into our places.
If I ask you how does your half look like (mathematical expression of the state), what would be the answer? (How does my half look like?)
If I apply some operation to my half like maybe applying pauli $X$ gate which flips qubits, would it change anything in your half?
What would the combined state look like after this operation?