There are roughly (I am not an expert in this) three things that make up a video file. A container format, then a video and audio encoding. MP4 is a container

There are roughly (I am not an expert in this) three things that make up a video file. A container format, then a video and audio encoding. MP4 is a container format. Think of it like a stream optimized zip file. Then inside this container, various video and audio codec actually do the work of compression their respective components. While MP4 is based on the MOV container format, I don't know if they are 1:1. So at minimum, all the contents would need to be unpacked and moved to the MP4 container format if the codecs were staying the same. That doesn't sound like what is happening here as that should be really quick. (If that's even possible). That's why parent was asking about codecs, if the MOV has video in MPEG-1 and Google wants it in H.264, it has to reencode the video stream.