I wonder, aren't virtually all "easily" available resources already dried up to such a degree that highly advanced/specialized equipment would be needed to extract whatever is left?
Parent post is probably referring more to energy resources. Gas and diesel degrade fairly quickly and unless you live in the middle east, aren't just flowing out of the ground anymore. Solar + BEVs might be an option, but only if the apocalypse scenario didn't involve an EMP, otherwise you better know your way around diesel engines and making biodiesel + live close to a hydroelectric plant.
Hydro, wind, and solar-thermal power is 100% renewable and can be made with 19th-century technology once you've worked out how to build a dynamo. There are some geographic restrictions but one of those power sources should be available in most regions. Plus most of the population has died off in a post-apocalyptic world, so your generators don't need to be nearly as big.
Almost all power is generated by heating water into steam, which can still be done by burning almost anything. It won't be very efficient, but you can do it - so traveling to the nearest coal mine/plant may be an effective energy source.
The most easiest thing, so, would be a Sterling engine then. Something I have to read up again, I'd love to build one with my kids. After all, it was the machinery we did hand-drawn technical 2D drawings for back at university before we moved to CAD.
Steel does have a major use in your skyscraper but concrete is perhaps the main player.
Conc is a wonderful and bloody complicated material and so is steel.
Steel is basically iron+(stuff) - Fe 'n' that. If you add small amounts of carbon to iron you get steel and depending on how you do it you transform iron (brittle, hard etc) to a material that is "tough". Tough generally means that it will resist stress/strain more and will fail gradually rather than catastrophically quickly. If you add some other elements, such as chromium you get stainless steel. I can't precis a three year degree into a paragraph but this gets you started!
Conc is a remarkable material, which we think was invented by the Romans. It sets and cures rather than "dries" so will will quite happily work underwater - provided you stop the constituents being carried away by currents. Setting conc involves an exothermic reaction so it heats up - too much in one pour can set itself on fire!
Add steel in the form of "rebar" to conc and you have a material that is nigh on magic in its properties but you do need to know what you are doing. You can simply put C section steel plates in your beams or run FeCr rod through and tension the nuts (lol)
Conc n steel are the modern building blocks of the modern world. I'd like to see a lot more wood ...
Folks interested in such material science topics should definitely read about Wootz steel[1] (Damascus steel[0]) and roman concrete [3]. Its very hard [2] to concoct a high fidelity version of these today, mainly because it is hard to get the trace constituents right.
I could try to dig out my college reading list (it was actually Plymouth Polytechnic - that's Plymouth, Devon, UK) but it was 30 odd years ago. Things have moved on a bit but not too much.
Do you have a general interest in Civil Engineering or a particular project in mind?
You will just have to strip materials from cars and buildings instead of digging them out of the ground.