Not sure how it works if you have a youtube account/are signed in but without an account/not signed in, if I remove after the ?, I get .....https://www.youtube.com/watch ... which is no use for anything. Genuinely interested to learn more about this.
Are you sharing the url or clicking the share button and getting a link?
I always just do a copy/paste of the URL. I just looked at a youtube video and saw there is an option to share a link. Seems that share option is where the extra question mark/tracking lurks.
I'll continue to do the URL copy/paste.