There are lights that seemingly never go out in some levels. Just divert the power going into them elsewhere using your own wiring/something of that nature, and you have a theoretically infinite power source, which will grow stronger the more you hook up to it. Some levels have areas so hot that you could turn that heat into very potent energy. And I'm sure you could use water wheels and whatever other sources you can think of, since just about everything that can exist exists there in various levels. Just scavenge levels for parts that already exist, such as the wiring I mentioned, and you have it very easy compared to the frontrooms starting out.
As for industry, people can theoretically enter the backrooms from any era, which means knowledge of extremely advanced, maybe even futuristic manufacturing and industry may already exist there in someone. There is theoretically infinite of pretty much any resource you could ever need in various levels, so that could be used in making various, much more advanced weaponry, defenses, and technologies for security and making life more convenient. The Frontrooms' Earth had limited resources, albeit a lot of them, so it would eventually run out and be exceeded by the backrooms' industry. Technology may even be able to do more there, since items like fire salt, liquid silence, and many other anomalous substances exist, which have been proven to be able to be mixed with various useful items to make them more effective.
It's also worth noting that aging is extremely slow after a point in the backrooms, thus people will have much more time in their lives to work on their technology before they have to let someone else take over.
While there is no currency to motivate people to have jobs and the like, the benefits of living safely and comfortably in a place with so many horrible ways to die may be a primary goal of many groups, so it seems more than likely that they would work together to achieve great things.
