The hook-up is all of it. Extensible arms with 2-way freedom of motion at one end, one or two way at the other, makes hook-up a one person job, and you can be pretty sloppy about locating the two vehicles, it still works when centering error is on the order of a foot.
Solid bar is almost always a two-person job, because someone has to hold the coupling above the ball, and direct the driver of whichever vehicle is being moved. Allowable error can't be much more than an inch if you want to get the coupling on the ball.
But a solid bar is a whole lot less expensive to fabricate, I've seen bars under $100 for lightweight tows, $200 can get you a really sturdy one.
There are some in-betweens, at in-between prices. The Stowmaster stays on the towed vehicle, and hooks over a ball, but because it has telescoping arms there is some more room for error, what you have changing length of two sides of a triangle. Someone really good at it can position alone, get out and lift the coupling over the ball.
I had a home-made on my Ranger, A-frame made of bar stock (a lot heavier than it needed to be) with a movable tongue, could move about two inches front and back, 3-4 inches side to side at the coupling. Once hooked up the rig had to be moved to latch it into position, then a bolt run through the movable part of the tongue to tie it all together safely. Enough trouble that I paid $400 for a used "almost new" Blue Ox Aladdin to replace it. Aladdin because there's enough aluminum in it to make it about 20 pounds lighter when I have to put it on or take it off the motorhome for winter storage.
