LOL! Yep...same issues nearly all of us faced with new rigs. The capped line will be to draw your antifreeze when winterizing. For your water heater (WH) bypass, it will almost always be right at your water heater. Two lines, with valves and a third line connecting the other two. The two lines, input and output will be closed. The third line which connects the two will be open, effectively bypassing the WH.
Be sure you drain and clean all antifreeze before you open up your Water heater...
And welcome! Enjoy the fun and ask anything you have questions about. We're all in this one to help each other.
Darren, Donna & The Wieners