Originally Posted by
mistermap
If it's a YYZ AC US to domestic connection, don't worry about Nexus. It's generally faster to follow the connection route (OSS) which lets you avoid going landside but has no Nexus. That should work for United to AC domestic too.
If OP has checked bags, sticking to all AC metal from the US connecting in YYZ or YUL is preferred to UA, since bags will be checked through. I believe with UA connecting to AC you will have to clear normal immigration to pick up the bags and put them back on the belt post-customs.