In AC system, current always flows alternatively, in both directions. Therefore talking about current is meanignless, and we should say "power flow". While the direction of current alternates back and forth 50/60 times a second, voltage alternates in sync and because power = voltage * current, direction of power does not alternate. E.g. +1A * +230V = +230W, and 10 milliseconds later -1A * -230V = still +230W. Direction of power is simply from balance of consumption and load as explained above.
If it is a 3-phase system, it's possible some of the phases export and some import at the very same time. And some types of loads can change pretty abruptly, too. So it is possible to import/export "at the same time" or "nearly at the same time". Whatever "net meter" exactly means depends, but I think at very least it would calculate net sum over all three phases, so that this doesn't matter. Usually "net metering" also implies summing production and consumption over some time period so that during that period, there is no simultaneous buy/sell (even if there physically is).
For example, you could sell every other second and buy every other by turning your electric stove on and off all the time. Here we are transitioning into 15-minute net interval. That would mean summing all those numbers together so that during 15 minutes, you only sold or bought, not both.
If Apartment7 bought the whole PV system and got permission to use common roof etc. or whatever (you get the point), then the role of other apartments is no different at all compared to the next house, and the next, and the next. They self-use what they can, and rest is sold to grid for common use by everybody on that grid (possibly spanning many countries!), and your other apartments are as much consumers as everybody else.