I dumpster dive for their fancy wood. When the cabinetry goes in at the end is when the dumpster contains the nice finishing wood.
If the scrap lumber is in a trash dumpster, especially if the dumpster is sitting on the public street, it’s probably fair game. But I would caution against going on the actual job site and taking lumber just because it looks like scrap. It would be better to ask the job super if you can take it first. Most of the time they will say yes and be glad to be rid of it. But also keep in mind that some refuse haulers give discounts or rebates on the amount of recyclable materials and scrap wood can be recycled to make things like particle board. They could also be concerned about liability should you get injured while climbing though a dumpster on their propery. So again, ask permission first.
With that being said, my late dad was a carpenter and built cabinet complete with a butcher block top - individual hardwood scraps, glued and using wooden dowels to make it, that he made nearly entirely from scrap woods off a job site. That was nearly 40 years ago and I’m still using it. : )
I’ve only ever taken from the dumpster or if clarified that a pile is scrap, cut offs, with the work crew.
Thus, the dumpsters (which are all on the property and not on the street) are fair game.
I only go when the workers are absent, and during daylight. I leave the blinkers activated on my truck so any observer will conclude I am not being sneaky. I also only go when there is no temp fencing, because that is de facto trespassing.
I have observed that the contractor is rarely on site - It's always the mexican workers and they have no idea who is who.
I want to build a shed. I am biding my time.