Both Leica TL2 and Sony A9 have 24.0 MP resolution sensors but Leica TL2's sensor is APS-C (23.6 x 15.7 mm ) and Sony A9's sensor is Full frame (35.6 x 23.8 mm ).
Since Sony A9's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Leica TL2.
Below you can see the and A9 sensor size comparison.
As seen above, Sony A9 has a 2.3x Larger sensor area than Leica TL2. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.