what will happen if i dont pay my icloud

Anything over the 5GB free space will be deleted, yes.

You need to pay until you downgrade your iCloud+ plan.

If you have an iCloud+ plan subscription, you will receive a monthly bill from Apple, check your email box.

👉 The subscription runs as long as you don't downgrade.

Your default payment method will be used first by Apple when you need to make a payment.  

If you have set your Apple Account balance as your default payment method and you have a CC / debit card, you pay from your default payment method.

If your valid payment method on file or Apple Account balance can't be charge, you can't download, update and/or purchase from Apple. If you try download, update and/or purchase, then you will see a message that says "Your account has been disabled in the App Store and iTunes", because there is a problem with your payment method on your account

Or -> you get a message 'Pending purchases', that indicate a 'purchase' that was unable to be charged successfully to your valid payment method on file or Apple Account balance, as such you are responsible for that purchase, and must allow it to successfully complete before anything else can be done.