A White Christmas … in SoCal
It's a Wonderful Life was filmed in black and white on 35mm film. There's a colorized version floating around out there that Frank Capra absolutely hated. ("Colorized" means that it wasn't originally in color; the color was added later). The classic black-and-white version is the one most commonly shown these days.
The Bedford Falls set actually wasn't built anywhere in upstate New York. It was created in the arid climate of Southern California on RKO Pictures' movie ranch, using some of the set pieces from a 1930s hit Western, Cimarron. The ranch was a sprawling 4-acre set, where Capra reconstructed much of an entire town. The set, which took two months to build, had a Main Street with 75+ buildings, a residential neighborhood, and 20 transplanted oak trees. Capra let dogs and cats roam around the set to make it look realistic. (Source)
The filming took place over three months during a heat wave in California. It was scorching throughout the production. Jimmy Stewart's sweat in the scene on the snowy bridge wasn't just a result of George's meltdown. It was 90+ degrees that day, and he was dressed for winter.
All of the snow used in the movie was fake, natch. Capra thought that the usual fake movie snow (a mix of crushed cornflakes painted white) was too crunchy and noisy, requiring those scenes be dubbed. Capra wanted live sound in the scenes, so they developed another, quieter snow made from water, soap flakes, and a foamy fire-fighting chemical, all blown through a wind machine. The film received an Academy Award acknowledgement for its technical advances for this new "snow." (Source)