What happens if you pick up a factory building that has machines inside it?? does the building remember it's interior configuration when you place it back down or are the items inside returned/deleted???

Everything stays as is, even storage, not sure if machines keep working in picked up building if it has power. You can make portable megabackpack with warehouses/storage in factorissimo building, you can have working logistic zones in the buildings too, the robots stay inside in their own miniworld and keep working.

The building no longer receives power from outside once picked up, but the machines inside will keep working (at least until the power buffer runs out). You can put an internal power source inside the building to make it keep working (and maybe even provide power to wherever you place it), but every vanilla power source requires either sunlight or lots of water, which means that you can't permanently power your pocket building without a huge water buffer or at least some more mods.

This was answered in the FAQ.

My answer is older than the FAQ, I rewrote the entire FAQ yesterday.

yes everything keeps working inless power is supplied from out side the building the power is then cut also any players in the building remain inside even in your invintory :D i mined my friend and stuck him in my quick bar then laughed he then nuked evereything and tryed to escape everything works if theres power but only personal robo ports can enter the building once in your invintory tardis factories are fun