Well what is tri tip beef? I had never heard of it. What part of the carcass does it come from?
It's the lower right triangle of the bottom sirloin.
Try this video from a butcher who explains all the cuts they get and where they come from on the steer
https://www.youtube.com/watch?v=WrOzwoMKzH4
The packers call it bottom sirloin butt. It comes off the bottom of the pelvis bone below the top sirloin. The top sirloin is an extension of the strip loin which is what the New York steaks are cut from. There are actually two in each steer, one on each side.
Bottom sirloin.
And the right carcass cooked the right way cannot be surpassed.