Video Output (Atari 7800)
The Atari 7800 has a dual video output. One for the 7800 games, one for the 2600 games.
The video output for 7800 games comes out of the Maria (U1) chip.
The video output for the 2600 games comes out of tia TIA (U8) chip.
Both signals go into an OR Gate (U3), which is controlled by the Maria pin 26 (!TIA Chip Select), which when low the OR gate outputs the TIA (2600) video signal, and when high outputs the Maria (7800) video signal.
OR Gate Mixing
Note that although for this specific revision of 7800 (PAL Rev C C300633-001) the U3 OR Gate is in this order, according to the schematics of presumably different versions, the U3 OR Gate has its pins in completely different order.
Be sure to check with an oscilloscope or multimeter which pins go back to the Maria and TIA chips for the LUMA and SYNC pins, as the TIA/Maria never change pins.
As you can see, the Maria for the 7800 video out sends LUM3/2/1 into the OR Gate, and the TIA sends LUM2/1/0, as the TIA is only 3 bit color, whereas the 7800 is 4 bit color.
So the upper 3 bits are OR'd and the LUM0 of the 7800 is always mixed into the final resistor ladder from the Maria LUM0 (pin 42).
When in 2600 mode, the Maria simply outputs a 0V on LUM0 to ignore it for TIA output.
The COL pin of Maria (pin 43) and the COL pin of TIA (pin 9), are both connected together through resistors (Maria through R16 4.3k and TIA through R17 4.7k).
This COL signal is then pulled toward ground via R2 470R, and goes through L1 47uH inductor and C8 47pF capacitor for DC blocking and mixing into the LUMA signal path.
When in TIA mode, Maria simply puts its COL pin into high impedance, and vice versa when in 7800 mode.
IMPORTANT: Unlike the Atari 2600 COL pin which outputs 5V logic and gets converted to a COL pulse through external circuitry, the TIA used on the Atari 7800 outputs already valid 350mV voltage level COL data.
Video Path Overview
Find below a visual representation of where the LUMA, COL and SYNC pins flow from the Maria and TIA, to the final composite output.
The LUMA and SYNC come out of the Maria and TIA, through the OR Gate, and to the resistor ladder near the top, and out to the final composite.
The COL data comes out of each of the TIA and Maria, through resistors and merge through an inductor and capacitor then into the final composite signal.