You write code in your own playground. So that if it hangs, or gets stuck waiting for a keyboard input or doesn't work, you don't spoil what the users are using. This might be a dev environment.

Then when you feel you've done a good job, it goes to testing where they check edge cases and what not, what if instead of entering your email, your grandma enters her phone no,etc. Or if going around clicking stuff in a partular order causes the code to get stuck in a loop. This might be the testing environment.

Then once this is done, it's pushed to the production environment. This is the code that runs on the servers or on your phone app. They might push updates to apps only to some people at a time so any potential damage is restricted.

They use git a program to manage version control so that code doesn't look like our homework assignments from school, ala, project 2_draft_final_1. So when you are done you commit the code to your latest version. Then you send out a pull request, telling your boss(senior/veteran coder) to look over and pull your latest code into something like the dev environment.