How does the fuel sending sending unit sent the signal to fuel guage on a 71 dart.

Sounds like your fuel gage is working. Down the road, consider replacing the factory 5 V Vreg on your dash with an electronic one. Search "voltage limiter" and "Plymouth" on ebay, ~$25. Your factory one is a little silver box w/ 3 male spades. They are notorious for failing, sometimes taking out the fuel and temp gages ($$$). If you have a tachometer (Rally dash), your Vreg is built into your fuel gage.

When you get your new sender, you will likely find that the new lock ring that comes with it is too small to get a tight fit. Some bend the tabs out, but most re-use their factory lock ring. While you have your tank out, a great time to clean rust and repaint the body, plus the tank. Also shake out the junk that fell in (pens, wedding rings, ...).