I'm beginning to get my head around this. theb1keguy etc. saw it right away.

The freehub that came with the wheel is designed to spin, but had probably rusted and got stuck. It's possible you might have got it spinning again with patience, WD-40 and advice but it's damaged now, so you'll need a new one.

The freehub is part of the hub, and normally the cassette of cogs comes off by removing a lock ring, then sliding the cogs off (or on some older hubs the smallest cog has to be uncrewed using chain whips and then the others slide off) leaving the outer, splined shell on the free hub as pictured. However somehow you broke the freehub, and you took the outer shell with the cassette, leaving the exposed inner parts on the hub. They can now be removed by undoing the interior allen bolt as explained and then sliding them off.

Then you need a new freehub body, and a new cassette.

How many cogs in the original cassette (it looks like 6 or 7), and what kind of shifters do you have? Did the original set up have the cassette held on the freehub by a lockring (newer hyperglide hub), or by screwing on the smallest cog (older uniglide hub)? It's important because if you replace the old cassette with a larger one you may have to "redish" the wheel to compensate for the increased width. Also depending on what kind of rear derailleur you have it may not handle a different range of gears than the old cassette had. What kind of bike is the wheel going into?