Why In a direct sampling (direct RF sampling) SDR, the antenna signal is filtered and amplified and then fed straight into a fast analog-to-digital converter, with no analog mixer or local oscillator ahead of it. All frequency conversion, filtering and demodulation then happen numerically in the DSP. This requires an ADC whose sample rate satisfies Nyquist for the highest frequency of interest, which is why direct sampling first became common at HF.
Watch out The choice about a switching mixer producing I and Q describes a quadrature sampling detector (Tayloe mixer), which is exactly the opposite approach because it does use a local oscillator to translate the signal down before digitizing. The source-code-to-object-code option is about compiling software, not signal handling.
Direct sampling = antenna straight to the ADC, no mixer, no LO. The math does the tuning.