What's the best way to make and test changes to an app's back-end?

I've never worked at a tech company. I built an app from scratch with a NodeJS API and MongoDB hosted on AWS.

I will start hiring back-end developers to work with me.

What is the best practice for making changes to the backend? All the .js files are store in a repository, but what's the best way to test changes to the API before shipping?

The app is live on the app store. Should I put a duplicate build on TestFlight and connect it to version_2.js files and test it like that?

Or is there a better way to do it? Curious about what industry folks typically do.