Terraform what happens to manually added resources during apply?

this is a general question and I am quite new to terraform, how does terraform handle manually added resources that are not present in the config files during apply?
What happens in the case that I created an infrastructure setup manually and just afterwards wrote the terraform config, I have no state file present so it tells me everything will be created, what happens with the resources that are already present under the defined names?

It has no knowledge of their presence and does nothing to them.

It depends on the remote system that Terraform is interacting with, and exactly how the Terraform provider involved has been written.

The name conflicts might be detected, resulting in errors.

Or, if the remote system uses the same API call for create and update, Terraform might issue what it thinks is a create, but actually modify the existing resources.

Or, if the remote system allows names to be duplicated, you might end up with two sets of objects.