How does code go from a Jr. Engineer’s computer to live on a business website?

I've only written software as a self-employed engineer. What this means is I lack a lot of knowledge about industry standards and how larger, multi-person teams work. I've always stored my work (completed or otherwise) on GitHub, but then still just push the local files from my computer to the live (prod) server.

One thing I've always been a little in the grey about is how code goes from a Jr Engineer's computer to prod?

I understand that the Jr would submit a push request, and then the code would be reviewed, but then what? Can you push code straight from GitHub onto the prod server? Or does the Sr/Lead handle that from their system through a terminal?

I've come to understand that there is commonly a "main" server, which is kind of like the staging area right before pushing to prod server?

How are side-projects and new features handled? They're just on a separate branch until merged later?

Thanks for any insight! Any videos you might know about, write-ups, whatever would all be very helpful.