Measuring out where they dig is an extremly delicate and precise task on any construction project. The construction teams are provided with highly detailed plans, precise to the millimetre. On a lot of sites these days they use laser alignment and other such tools to pick out the precise points. They will then mark these and begin digging and piling for foundations.
Rebar is used in most large concrete products. It isn't used to cast the cement or anything like that, but instead strengthens the solid cement, hence its name Reinforcing Bar. Normally, builders will dig a hole, they will then place rebar inside the hole, in the middle. They will then pour the cement into the hole and cover the rebar. It's then left to set inside the cement.
You can see it here. Eventually that hole will probably all be filled with cement, and the rebar left inside it. Welding it into boxes and cubes is a useful way to save time when adding rebar, as they can just drop on large piece in and fill. Alternativly they can layer the rebar in sheets while they pour like so:
But this takes much longer as it means they have to cut the bar, pour a bit of cement, place a sheet in, pour some more, place a sheet, pour a bit, you get the idea