Update content of .env during action

SO for the life of me I cannot understand why I cant find best practice on this matter

Real Simple - I am going to input a few Secrets into the repo
Then I want to input those secrets during build into a file in my repo (.env for Vue.JS)

How do you do this?

According to your comments, you will add few secrets on the Secrets page of your repository, then you want to export these secrets into a .env file in batch in the workflow. Right?
If so, maybe you can try to use the secrets context to do this.

- name: view the secrets context
  shell: bash
  run: echo "$SECRETS_CONTEXT"
    SECRETS_CONTEXT: ${{ toJson(secrets) }}

In the secrets context (JSON type), all the secrets you have added on the Secrets page of your repository will be listed, and it also will list the GITHUB_TOKEN.
For example:

Then you can try to export all the secrets (key-value pairs) from the secrets context into the .env file.