Q: What happens if u wear on the Apple Watch so loose?

Hi

Apple recommends wearing Apple Watch comfortably on the top of your wrist, neither too loose nor too tight and with room for your skin to breathe.

If you wear Apple Watch too loosely, the watch may not stay in place, the sensors may not be able to perform as effectively and the band may cause rubbing. For example: With Wrist Detection enabled, your watch may repeatedly lock whilst still on your wrist, due to the watch losing contact with your skin and mistakenly believing that it has been removed from your wrist. Haptic feedback (taps on your wrist) may also be less noticeable. The performance of the heart rate sensor may also be affected.

Wearing the watch too tightly may lead to skin irritation and/or discomfort.

More information:

Wearing Apple Watch - Apple Support