What happens is that everything starting Monday is pushed back two weeks. Stock markets close. Non-essential workers stay home. Non-essentially businesses close.
During the two weeks, government pays for everyone's utilities, food, rent, and so on, both for people and companies.
The idea is to as much as possible pause the normal economy for a couple of weeks, and then resume it hopefully pretty much right where it left off, with the government seeing to everyone's needs during the economic pause.
Do this once a year as a readiness exercise, probably trying to time it around when that year's flu season is getting serious so that it may also help reduce the intensity of flu season.
Over the years, the government should over fund this to build up a cushion, so that whenever the next pandemic comes around they have enough to do longer than a two week pause if necessary.
Is it a guarantee that 100.0000000000% of the population will eliminate the virus with their own immune system in three weeks? If one or two people with poor immune systems carry around the virus for one day past the quarantine then we we would very quickly have gotten back to where we started.
It's also implausible to quarantine everyone for the same three week period. Are we just supposed to let anyone with a major health crisis die at home if it happens mid-quarantine? What would we do about the millions of people who require round the clock care in nursing facilities, etc.?
Additionally, I'm willing to bet that if we dedicated 5.8% of worldwide GDP (21 days out of 365) to fighting viruses, the results would be much more successful and less invasive.