[CivicSpoon]

Pistons and transmissions are interchangeable between all.

Almost every part of the B18B and B18A are interchangeable between each other. But between the B18B/A and B18C's (B18C1 and B18C5); the rods, cranks, heads, even the physical block heights are different.
There is a way to put the B18C1 and B18C5 heads onto B18B/A blocks, but it's not a direct bolt on thing. There are kits available (and instructions online) so you can do it.

The heads are slightly different between the B18C1 and the B18C5, the intake manifold bolt pattern is completely different (though the B18C5, B16A, and B16B all have the same heads with different cams and valvetrain). Though, the B18C's share the same rod length, cams are interchangeable, valvetrain is as well, and you can swap the heads (B18C5 head bolts directly to the B18C1 block and vice versa).

Those are some of the major parts that are or aren't interchangeable. If there are any other specific parts you're wondering about, please feel free to list them and I'm sure someone will be able to help you out.