Firebase - storing the API key used for authentication

As you already know, you need to authenticate with the remote API to consume it right? How are you currently authenticating? Where are you storing the API key used for authenticating?
Would you find useful a very easy to use tool that can secure your API key and get yourself an API gateway to handle all the requests?

For example, this tool would ask you your API key and API link to be consumed, then this tool will handle the authentication and will give you back a link (API gateway) that you can use to consume the API safely.

Firebase config (runtime env variables) or for anything too secure you can use or identity platform