Also what adds to the problem is that I'm an introvert
From an introvert to another, I can tell you this is a bad excuse. If you have gone through college and finally made it to the working market and you still are COMPLETELY an introvert then that is a problem. I noticed I had problems with this when I was younger and worked on it myself. Actually during interviews when I am asked my weakness this is mostly what I use, as I give the example of what I've done to improve it. In my case, it was performing street magic. In your case can be as simple as joining in a conversation during lunch. You need to fit in as you will be spending 1/3 of your life with those people from now on...
There has been almost zero help from my colleagues or manager, who
expect me to know it all by myself (As in how to behave, how to ask
for help or work).
You are the new guy here. They all know their systems, what their code looks like, their shortcomings and maybe even industry jargon if this is a dev team for a specific market. It's like when I visited Germany and almost got ran over by a bike in the bike lane and the German guy shouted abuse at me. People will undoubtedly expect you to know, but this is out of pure habit. Don't be afraid to ask questions.
As said earlier, i have zero experience of the life at jobs.
We've all been there. Secret: They've been there too!
But the real question is, i have been waiting too long for some real
work or some tasks.
I started working where I am 3 months ago (nearly) and as a business project manager I was hired to improve all the processes and services. I am instead coding automation for my department. Problem? I'm still waiting for licenses to be paid for software I need to use, so technically I can go through 2/3 days coding and 7 days of....stack overflow, youtube tutorials, udemy,codeacademy....it is boring yes...sometimes that will happen, being in the IT industry there is one thing that can happen: As a dev this won't apply as much but it will definitely apply as an app support analyst (my previous role).
You are only needed when things go wrong.
This CAN happen as well to you, but not as often. It is normal to have some downtime at work sometimes. This is where the introvert part needs to disappear. You can pick up work from another college, or simply just check their code, you may find new things and learn new things.
Most importantly, ENJOY! You are now earning some money...find something to invest it in before you learn to waste it...set yourself some achievable career goals and set yourself your next career step and go for it!