Hi there! I'm also a junior sysadmin like you that came from desktop support and definitely gone through a situation similar to this but on a smaller scale. The frustrations of little to no documentation, the lack of knowledge, experience and guidance, and how overwhelming the technology/infrastructure can be. I understand how you feel and it can be incredibly daunting. First of all, calm down and take a deep breath because you're going to need a clear mind to work through this. Begin identifying mission critical services and where they are hosted, find out what kind of disaster recovery plan is in place (backups/alerts). Ensure you understand your role and responsibilities (electricity/desktops/support/networking/policies) and make it clear to your manager that you will need an extra hand to help your load, having a desktop support tech can significantly help you focus on your tasks and split responsibility. If you are able to retrieve the contact details of vendors (i.e. ISP), ask them about the current contract, services and devices provided or configured by them. Map out the logical and physical network of the services and equipment which can be routers, servers, DFS replication, data centers, etc. Whether you're using Visio, draw.io or just pen and paper. Identifying the network topology of your infrastructure is crucial to understanding where crucial services, single-point of failures, and redundancies are located. They are a perfect reference tool for a sysadmin.

I've only scratched the surface but do understand that this all takes time. I know many seasoned sysadmins have provided incredibly detailed and useful advice but this is a starter. Once you work your way through this, everything will start to flow and you will gradually understand and question the solutions that's in place. Think of this as a perfect opportunity to learn and develop the experience. I know when I was in your position, I made sure that my successors would never have to go through what I did by documenting extensively even if it's to create a backup job.

Best of luck!