The ones I have use 3d printed frames, the fire control group and barrels are metal with 3d printed pieces for making the rifling.
All the parts that take repeated heavy abuse are reinforced with extra thickness or different infil, but by weight I'd say it's about 50/50 metal/plastic.
The 3d printed lowers are quite basic, and since they aren't designed to take a ton of stress anyway, it's not really hard to find a decent design.
All my parts are printed in pla+, and I do minimal work afterwards to make things perfect, only what is necessary for the mechanical parts to cycle properly.
I actually haven't been keeping up the last few years, I wouldn't be surprised if there are even better methods than the ones I've used.
The most reliable one I have uses an AR style fire control group, and large metal pieces for the bolt, and pre-hardened hydraulic tubing for the barrel.
There's plenty of metal in it, all held together via 3d printed parts and frame
The only things I needed to buy that I would consider "from a firearm" or "from a gun store" would be the fire control group. Everything else was bought from McMaster Carr or local hardware stores.
Spot on with the woodworking. I've made a couple stocks for my grandfather's old broken long rifles. It's just more time consuming. (also my 3d printer isn't that long)