The part that says either ?si= or &si= is only for tracking. You can remove this part of the link and it will still function just the same. It makes the link shorter and also helps protect your privacy. The only important part of a YouTube URL is the video ID. There are two types of URLs:
For this one you can remove everything after and including the ampersand if present.
For this one you can remove everything after and including the question mark.
Note the exception is if you want to intentionally leave in playlist information you might want to keep that part.
I’m not entirely sure how well it works since I don’t often send links to others, but I use a uBlock Origin filter list called Actually Legitimate URL Shortener Tool. It may help those who don’t want to do all of this manually?