Hosting project using firebase [duplicate]

I have a question, I made a website using firebase database and React, so I use an API KEY.

Now I want to host this website online but the problem is that I don't know how to hide my API KEY from my code.

I used a .env file, but how does it work in production then? How do the production ready code knows where to access the key?

Thanks by advance.

Edit:

The anwser is there: Is it safe to expose Firebase apiKey to the public?