Continuity mistake: Rick Malverne has handcuffs on when J'onn goes in as his dad. When he figures out it was J'onn, he tells them they have "23 hours and 14 minutes," looks at his watch and the cuffs are gone. (00:21:30 - 00:25:00)
Continuity mistake: Right after Alex is freed from the water tank, in the shots of Maggie holding her, Alex's clothes are bone dry.
Answer: For one, it's usually cheaper to film in Vancouver because of the tax breaks. But all the CW Arrowverse shows (with the exception of "Black Lighting") are/were filmed in Vancouver. So it makes it easier and cheaper for sharing set designs, crossovers, etc ("Black Lightning" filmed in Vancouver during its crossover). Originally "Supergirl" wasn't part of the Arrowverse when it was on CBS, but it was too expensive to produce and wasn't reaching the wider audience the CW shows had. So CBS moved it to its sister company, The CW.
Bishop73