Hey CA, welcome to the forum!
Shooting heads are short- 30 feet or so -- compared to a full length fly line at 90 feet or more.
Typically you’d match up a rod with a shooting head the equivalent grain weight of a normal fly line 2 line weights heavier, although "tuning" heads to rods and individual casting strokes is something of an art in itself.
You false cast with just the shooting head out side the tip--- and it shoots the lighter running line out when you release the head for a cast.
There are a couple of different types of running line, and they all have their fans:
Thin coated stuff similar to a 2 weight level taper fly line
Specialty mono like Amnesia
Braided stuff made by Airflo and others
If you’re just starting out with them, a fly line type would be the easiest to manage,. (Slack running line tends to blow around a lot and can be difficult to control, especially without a “shooting basket”. The mono, both specialty stuff like Amnesia and coated versions can be difficult to control, cut your wet hands, Next best (imho) would be a braided type (a lot lighter so more distance than fly line types). You want something like 60 feet of running line (or 90 if you’re an optimist) attached to your shooting head, and if there’s room left on your reel, fill it with whatever you’d normally use as backing.
Here's a good link if you want to read up a bit and/or make your own:
Shooting heads DIY - 1st section - preparing the lines - Global FlyFisher