https://www.hollywoodreporter.com/news/as-hollywood-maps-return-new-set-protocols-take-center-stage-1292071
Here's the part that relates to some of the Arrowverse shows like Supergirl that shoot in Vancouver.
For productions that are willing to work in other territories, there are other hurdles standing in the way, like the fact that it's unclear when certain closed borders will open up again for noncitizens. In Canada, for instance, Toronto Mayor John Tory and his city's public health officials say they are actively working with other levels of government nationwide on plans to reopen the U.S.-Canadian border, followed by a lifting of 14-day quarantine orders, to help get the film sector back up and running. "If you continue this indefinitely," he says of the quarantine period, "it will impair the number of people who can come here to readily pick up production of a TV show or film because they'd have to count themselves out of action for two weeks." Williamson says series leads have already begun to return to Vancouver to sit out their quarantines so that they can be ready to jump into production on day one, whenever that might be.
The whole article is worth a read.