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 know its meant to do it, but I have no idea how it works on mobile. Which is annoying, because its most useful on mobile.