You fine-tune the open-weight model without access to the original weights. If you want to train it from scratch you need data, yes. But presumably if you are

You fine-tune the open-weight model without access to the original weights. If you want to train it from scratch you need data, yes. But presumably if you are doing that there is a reason you want to do it. You lose nothing without access to the original data - you can do every single modification without it. That is unlike open source where you (mostly) need to source code to modify it beyond what the original designed originally thought.