How to upload to a specific YouTube channel of the same account? (YouTube Data API)
Checkout this:
https://developers.google.com/youtube/v3/docs/videos/insert
And notice the onBehalfOfContentOwner and the onBehalfOfContentOwnerChannel parameters for the request, i think this is what you need.
This parameter is intended for YouTube content partners that own and manage many different YouTube channels. It allows content owners to authenticate once and perform actions on behalf of the channel specified in the parameter value, without having to provide authentication credentials for each separate channel.