How do you keep bedbugs out of your apt when another unit has them in multi-unit building?

It depends on how close to your apartment the bedbug infested apartment is. The most obvious precaution is under no circumstances buy or borrow anything from that apartment. In particular whatever you do, do NOT buy or borrow any furniture any bedlinen or any mattresses. If you do you will almost certainly get a bedbug problem as there will almost certainly be unhatched bedbug eggs in such items. Similarly do not let anyone living in the infested apartment or anybody that has visited it, such as the landlord come into your apartment after being in there, because it is possible he or she will carry bedbug eggs or even one or more live bedbugs in his clothes or shoes or some such item with him. If it is very close another danger to you is that bedbugs find hosts by human breath emitted by some person when asleep. So if a nearby apartment has an infestation do not leave any Windows or doors open facing there when somebody is asleep anywhere in your apartment Bedbugs can detect human breath of anybody asleep as far away as 50 metres and once they detect it, if where they are is already over crowded with other bedbugs, they might try to move to somewhere else, like your apartment after being enticed by the sleeper's breath of anybody in bed in your apartment. It makes no difference if the breath if from a man or woman, adult or child, all are equally enticing to an adult bedbug or bedbug nymph wanting food.