How to generate file and re-use its content with terrafrom

Instead of generating a file using an external command and then reading it in, I would suggest to use the Terraform tls provider to generate the key within Terraform itself, using tls_private_key:

terraform {
  required_providers {
    tls = {
      source = "hashicorp/tls"
    }
  }
}

resource "tls_private_key" "example" {
  algorithm = "RSA"
  rsa_bits  = 4096
}

The tls_private_key resource type exports two attributes that are equivalent to the two files you were intending to read in your example:

• tls_private_key.example.private_key_pem: the private key in PEM format
• tls_private_key.example.public_key_openssh: the public key in the format OpenSSH expects to find in .ssh/authorized_keys.

Please note the warning in the tls_private_key documentation that using this resource will cause the private key data to be saved in your Terraform state, and so you should protect that state data accordingly. That would also have been true for your approach of reading files from disk using data resources, because any value Terraform has available for use in expressions must always be stored in the state.