For example, Spielzeug refers to things played with, or toys. In contrast, Flugzeug doesn’t refer to things that fly, but refers to only airplanes. Do -zeug words usually fall either the category of specific objects, or do they usually refer to categories?
-zeug used to be a collective noun, referring to a set of things. This is still the main use:
Bettzeug, Badezeug, Flickzeug, Grünzeug, Handwerkszeug, Regenzeug, Schlagzeug, Schreibzeug, Schuhputzzeug, Spielzeug, Strickzeug, Werkzeug, Zaumzeug
Sometimes, however, this set became a single thing in the public mind, while people still kept using -zeug for it. Take Feuerzeug (lighter) as an example: these days it’s a simple tool, but it used to be steel and flint and tinder and whatnot: the whole Feuerzeug.
Apart from that I can only think of Fahrzeug and Flugzeug, which both are complex things consisting of engine, gears, chassis, etc.
The previous answers are already good but I too want to add something. While in today’s language Zeug means stuff (e.g. “Räum dein Zeug weg!” = “Clean up your stuff!”) it used to have the connotation of “the things you need to do something”.
There still is a saying: “das Zeug dazu haben” (“to have the stuff to do something”), which means to have the guts or the skills to do something. Also the German word for arsenal or armory is das Zeughaus, so the word Zeug did not always have the vague and sometimes disrespectful undertone it has today (which it only has as a separate word, not in composed words). Maybe a better translation for the former meaning of the word would be “set of tools”.
The term Flugzeug may be derived from Fahrzeug, where it would be easier to understand why something like a car may traditionally be described as a set of tools (if you think of something like a horse cart, which in a way is the set of tools you need to “drive”, rather than ride, a horse).
Some additional information to complement Ingmar’s good answer. Zeug translates to stuff, and you will get a perfect intuition for what’s going on if you just translate it that way. To apply this to your and Ingmar's examples:
- Spielzeug = playstuff = stuff to play with = toy(s)
- Flugzeug = flightstuff = stuff to fly with = plane
- Bettzeug = bedstuff = stuff to make a bed with = bedclothes
- Badezeug = bathingstuff = stuff to bathe with/in = swimming gear
- Flickzeug = mendstuff = stuff to mend with = repair kit(s)
- Grünzeug = greenstuf = stuff that is green = greenery
- Handwerkszeug = craftstuff = tools [of the trade]
- Regenzeug = rainstuff = stuff needed when it rains = wet weather gear
- Schlagzeug = beatstuff = percussion
- Schreibzeug = writestuff = writing utensils
- Schuhputzzeug = shoecleanstuff = shoe cleaning things
- Strickzeug = knitstuff = [the] knitting
- Werkzeug = workstuff = tool(s)
- Zaumzeug = bridlestuff = [horse] harness
- Feuerzeug = firestuff = lighter
- Fahrzeug = drivestuff = vehicle
(I am not sure if this way of forming German words is still productive, but as my pseudo-English translations demonstrate, making up your own compound words this way is a better strategy than saying Ding (thing).)
For some values of x, x-stuff typically exists in sets that can be used separately from others. In those cases, the mass noun x-stuff can get a homonymous countable noun, as in the case of Spielzeug. The singular is then often ambiguous about whether it’s the uncountable mass noun or the singular of the countable one. Over time, the uncountable mass noun may disappear or may be used as a singular noun for a set.