Question Question about using 2 screens

[Teknoman2]

Its not a problem that needs a solution, just curiosity. I have a Dell G2724D and a very old LG 22M35. The first is connected through DP, the other only has VGA and i use a VGA to DP adapter. Now what i find curious is that when i turn on the PC it uses by default the old LG (for bios, windows loading screen etc) and only after windows loads it switches to the Dell as the primary. I don't really care to change anything i'm just curious if anyone can explain how and why the PC chooses which one to use until windows takes over.