I'll try a "primer" and see if that helps.
The hitch has a base that mounts through the truck bed to the frame. This is permanent and some are designed to be less obtrusive than others, though most new ones are pretty much minimized. Depending on what you may want to carry when the hitch is removed, you might select a different design of base - some go crosswise and some lengthwise. There is no "best" - it's personal preference.
The side supports of the hitch attach to the base and then the "fifth wheel", a large slotted plate, attaches to the sides. In some hitches the fifth wheel crosspiece is detachable from the side supports, which makes it somewhat easier to remove the hitch because the weight and size of the components are smaller. None of this is important if you never remove the hitch.
The fifth wheel plate is what the trailer King Pin rests on and latches into. The king pin is simply a very robust knob that protrudes down from the flat plate on the trailer's nose piece. It's what gets pulled or pushed by the tow vehicle. The weight of the trailer front end (the kingpin or hitch weight) is carried by the matching circular flat plates, one on the trailer and one on the truck's fifth wheel hitch. These plates area standard size in Rvs, so matching them up is not a problem. There will be a cross bar on the fifth wheel hitch that latches behind the kingpin when it is in the hitch. A slot in the king pin prevents the pin from lifting up and out once the latch is engaged.
The fifth wheel plate is mounted such that it can swivel in the fore and aft direction, providing a flexible joint that allows the trailer to dip or rise without lifting the trucks rear wheels off the road. Better hitches also "rock" from side to side, which allows the trailer to lean without twisting the truck or lifting one wheel off the road.
The slide mechanism (which is not needed on all truck/trailer combinatons) may be manual or automatic. It is used only when backing up and is needed only if the trailer front end could strike the back of the cab in an extreme turn. Many, perhaps even most, will not hit anyway, but lots of folks would rather be safe than sorry. An automatic slide engages by itself whenever you back up. a Manual slide requires that you get out and move a lever when you want it to slide back.
Some hitches now offer air springs (bags) to help cushion the interaction between truck and trailer. These can also be easily added afterwards, so it is not necessary to decide on that up front. The air spring can be mounted to either the trailer side of the hitch or the truck side. I'd probably wait to see if this extra goodie was needed.
Hope this helps - ask further questions as needed.
I don't think there are any "bad" fifth wheel hitches on the market, but perhaps some are more convenient than others. I'm somewhat out of date on actual experience, so I'll let current fifth wheel owners suggest their favorite brand. We had an RBW Little Rocker on our fiver and it was an advanced design back in 1999. Now many brands have similar features.