Are you joking OP? I can’t believe anyone would genuinely think the entire country could do this, never mind the entire world.
It would be virtually impossible in this country- essential and emergency services, utilities etc would all have to keep going- as would many essential industries which cannot be shut down and restarted quickly due to supply chain issues. Even isolating people at work would not help- if patients were going in and out of hospital or carers going between clients, for instance, there will still be potential for spread. Many workplaces are not set up for residential working and lack even basics above a toilet. Even where there are some facilities, probably not enough for the entire workforce to stay. Who is going to look after the children/dependents or even pets of these people who are going to be forced to stay at work for the duration of this period? Who is going to pay the loss of income from business forcibly shut down for this period?
So, unlikely to be possible in this country. BUT do you really have such little understanding of the conditions in the rest of the world that you cannot see that this is even less possible in many countries? Do you really think the infrastructure in the developing world would facilitate staying at home for 3 weeks (3 weeks would probably not be sufficient, by the way)?
Many people don’t have reliable (or any) access to running water so they have to travel to collect it, alongside many others. They often live in crowded accommodation where isolation is impossible. Nor do they have access to weeks worth of food at a time- either because the supply is not there or because they would not have the funds to buy sufficient quantities of food to last weeks (and no way to store most of it, even if they were given it). Few, if any, of these countries have a welfare state- if you don’t work, then you don’t earn any money- what do you expect these people do? Starve? Even if they were given food for 3 weeks of isolation, and the means to store and cook it, what would they do after that? I am genuinely astonished at the ignorance on display.