Really simple. What is the first thing you do when you have a web app? Is there any comment that can help you? Where would someone post public available code? Google is your friend on this one.
Things get creative here. You are in, you have a shell. What is the first place you will go? Get the clue as valuable. How can you execute things that are not in other’s home dir?
Here I was rabbitholed for too long. Enumerate the processes and understand what is being loaded and with which privileges. Look for something that executes everytime you log in.
It is kind of non motivating to see more expert people taking 30 min, 1 hour on this box (at least on comments), so I won’t hide this box took me about 6 enjoy it at your own pace!