Recently the channel https://www.youtube.com/c/trahan/ created a YouTube banner that auto updates every 5 minutes with a new one displaying how much he has donated to charity. I want to create a similar banner but with time until next upload. Is this possible, I'm guessing with some kind of script?
A channel banner is an image with no logic of its own. You cannot create a banner that updates conditionally.
You can use the YouTube API to set a channel banner, but this will require you to obtain an API key and to write a script/executable/etc that calls the API to update the channel banner with a new image when appropriate.