The Qiskit documentation on VQE describes two of the ansatz as "rotations with entanglements". The rotation gates are more or less clear, but the documentation doesn't mention what gate is used for entanglement. I suspect they use something like $\exp(-i \alpha Z_1 Z_2)$, but what exactly?
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Looks like the entangler gates are controlled-PHASE gates, at least that is mentioned in the Qiskit tutorial:
$$ c\text{PHASE} = \begin{pmatrix} 1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\ 0 & 0 & 1 & 0 \\ 0 & 0 & 0 & i \\ \end{pmatrix} $$
However, in hardware, one also uses the drift Hamiltonian $U_{\text{ENT}}=\exp(-iH_0 \tau)$, which naturally entangles all qubits.