The video encoding for generating composite and RGB video data from the VDP chip is the CXA1145 video encoder.

The CXA1145 is a super common encoder found on many retro consoles.

Look for it near the AV port on your Mega Drive, and here are the pinouts (Image is from a Neo Geo AES but all pinouts are identical).

The CXA1145 slightly amplifies the input RGB signals by around 40%. As an example, a blue in signal pulsing at 0.66V will output at 0.93V.