The Freehub incorporates the ratchet mechanism into the hub body (although the ratchet mechanism is still replaceable).
When you wear out the sprockets on a Freehub, you replace the sprockets only, not the ratchet mechanism (which typically lasts much longer than the sprockets).
The lockring has a normal right-hand thread: turn clockwise to tighten it.
To remove the lockring, you need to turn it counterclockwise, but then the cassette will freewheel, so you need a chain whip to hold the cassette.
Traditional rear hubs came with a standardized set of threads to which a standard freewheel/sprocket cluster could be screwed on.
This allowed any brand of freewheel to be mounted on any brand of hub.
The older Shimano cassette sprockets used a "twist-tooth" design, called "Uniglide." They had 9 identical splines (tabs) that would slide into matching grooves on the Freehub body.
You'll still need a threaded Uniglide sprocket for the top-gear position.
Supplies of these are getting scanty, though it is possible to grind a worn sprocket so it will work with a new chain, or use a 3/32" track sprocket -- the threading is the same, though you may have to grind down the flange or install the sprocket backward and use a spacer.
Spacer washers would fit between each pair of sprockets.
5- and 6-speeds used 3.65 mm spacers, 7-speed generally 3.15 mm, 8-speed 3.0 mm.