Can’t recall where I heard this, but I’m pretty sure the si=… is tracking information that associates the link with the user who shared it.

Oh absolutely, I am just wondering _what_ does it contain.

YouTube video ids are just integers in a base-64 encoding, modified to be URL safe.

Interesting. Any examples? I mean, I can probably reverse-engineer something myself but just curious.

I am much more interested in the `si` parameter.. but I am fairly sure nobody outside of Google knows what it is exactly.