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.
If you’re on Android, URLCheck is an extremely handy utility for cleaning up shared links.
While this is a handy tool, I actually like cleaning up URLs manually. You see a lot of interesting and sometimes malicious stuff in there, like tracking IDs, campaign names, source names, app names etc. It’s kinda rewarding to remove all that stuff and seeing the link is still working.
This utility breaks each one out so you can see them more easily, and selectively remove only the ones you want. It also allows you to directly edit the link manually at the same time, so you’ve got both the full control when you want it, and the convenience when you don’t feel like screwing around. I’d suggest giving it a try before you knock it - it’s quite powerful & flexible. Heck, it’ll even check if what’s left with the server & return the status code without you having to actually try loading the page.