The QuantumCircuit.append method looks as follows

    def append(
        self,
        instruction: Operation | CircuitInstruction,
        qargs: Sequence[QubitSpecifier] | None = None,
        cargs: Sequence[ClbitSpecifier] | None = None,
        *,
        copy: bool = True,
    ) -> InstructionSet:

with the instruction being appended to a quantum circuit being of type Operation | CircuitInstruction.

In practice, however, we can add also anything that has a to_instruction method (see also this comment). It would be nice to reflect this functionality in the type hints and/or in the following docstring.