Heterodyne modeling confusion in SDR
A negative frequency is given by the complex signal $e^{-j2 \pi f_c t}$ for which the physical implementation requires two real signal paths, which is made clearer from Euler's formula:
$$e^{-j2 \pi f_c t} = \cos(2 \pi f_c t) + j \sin(2 \pi f_c t)$$
So if the Local Oscillator is generated with sine and cosine outputs, and the multiplier is a complex multiplier (in this case it may have a real input so would result in two real  multipliers) then the receiver can be implemented as described.
Note that a full complex multiplier requires four real multipliers:
$$(I_1+jQ_1)(I_2+jQ_2) = (I_1 I_2 - Q_1 Q_2) +  j(I_1Q_2- I_2Q_1)$$
In this case the input from the LNA is real, representing $I_1$ in the formula above, while the input from the Local Oscillator is complex, representing $I_2+jQ_2$ and the product is:
$$(I_1)(I_2+jQ_2) = (I_1 I_2) +  j(I_1Q_2)$$
With the complex output as indicated in the block diagram.
Note I detail this further and the implications of the real versus imaginary input after the LNA in this post: Frequency shifting of a quadrature mixed signal (for imaginary input the output of the LNA would be fed into a Hilbert Transform and then the multiplier would be a full complex multiplier using four real multipliers).