“And how easy would it be to change a 49399797 field key to, say, 49399796, making an entire sub-structure of advertisement and tracking information suddenly unavailable?”
YouTube ads come from the same domain as the videos, so Pi-hole can’t touch them. Google ships them to the Apple TV as one Protobuf field, and changing the field number by one makes the app ignore the whole ad and tracking block. The catch is a pfSense box, a decrypting proxy and a fake-but-trusted CA certificate on every device. Google built a format that tolerates unknown fields and then hung its revenue on one of them.