I buy second hand phone but it is i cloud locked with ios15 how can i unlock it

You can't. Only the owner of the account used to lock it can remove the activation lock.

Talk to whoever you got it from.

If you cannot get the correct contact details to remove the Activation Lock, then that iPhone is useless to you. There is nothing else that you can do.

Axel F.

What if I no longer have the owners contact details?