Youtube isn't the only platform where Google does video transcoding. I don't know them all, but here are a few other places where video plays a part:
Meet - I'm guessing for participates that on are different devices (desktop, android, ios) and depending on their bandwidth will get different video feed quality. Though, the real-time nature of this may not work as well? Though, Meet has a live stream[0] feature for when your meeting is over 250 people, which gives you a youtube-like player, so this likely is transcoded.
Duo - more video chat.
Photos - when you watch a photos video stored at google (or share it with someone), it will likely be transcoded.
Video Ads. I'd guess these are all pre-processed for every platform type for effecient delivery. While these are mainly on youtube, they show up on other platforms as well.
Play Movies.
Nest cameras. This is a 24-stream of data to the cloud that some people pay to have X days of video saved.
[0] https://support.google.com/meet/answer/9308630?co=GENIE.Plat...