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The port pins of your Nano can deliver current in a quite limited range, and six LEDs need a lot more to shine brightly. The transistors are amplifying the available current.
However, 4k7 resistors in series with each LED will not allow enough current to let them shine well. Reduce the resistors by a factor of at least 10. Read the LEDs' data sheet and apply ...
2
The transistors are connected as an emitter follower, not as a switch. The transistors are acting as current amplifiers. The emitter voltage will rise to approximately the voltage on its base - about 0.7V. That places the voltage at about (5 - 0.7 = 4.3V) at the emitter. Since the collectors of transistors are powered by Vin I would guess you have at least 6 ...
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