Matte painting.
I get free to use photos, cut them together, and get them on the somewhat same saturation/color nuance.
The inserted references have to be blurred (with grauca-something filter) or lowered density.
Paint over them and make them somewhat fit into the picture. It helps to have some props/strong light/characters that divert attention from the background.
When done use the blur filter on the background again.
Works better with dark or muted colored backgrounds.