Pistol case and rifle case resizing are a bit different.
The body of a bottleneck rifle case is resized to the proper specs that will fit into a chamber, however, the neck must be resized to a small spec and therefore that area gets a bit more resizing. As the case is pushed up into the die, a deprime stem with expander ball (some have a tapered "ball") goes inside the case and 1) pushes out (deprimes) the old primer, and 2) on the way out, the expander ball expands the neck to a size that is about .002 smaller than the bullet. This gives "neck tension" and holds the bullet in place prior to firing. This is normally done all in one die.
A straight-wall pistol case has to be treated a bit differently. The first die will "squeeze" or resize the case to a bit smaller than chamber specs except down toward the case head which will be to specs. Undersizing must be done in order to hold the bullet. The second die, the expander/flare die will expand the case back out to specs plus flare the case mouth should you desire a flare to facilitate bullet seating.
As for seating/crimp, it's a matter of preference more than anything. If you want to seat and crimp in the same die, then that's what you buy. Should you want separate seat and crimp operations, then you can buy the addition die from most companies or buy from a company that gives you the dies in one package.
Personally, I've always sized and crimped with the same die when I crimp which isn't all that often. I've done it that way for a long time and just don't see the need to change. Now, I will say that I do crimp hot loaded pistol cartridges like .357 and .44 Mag. I don't crimp any of my rifle cartridges, but I do know what the fit between bullet and case neck is and have tested those enough to know set-back won't be a problem.