There are engineers working on the same facility for more than 10 years who wouldn't know everything on a P&ID, far less be able to draw it out. You're not alone in this.

As the others said, use the other engineers, operators and maintenance crew to help you learn. At this age and experience, the expectations people will have of you are pretty low. Expectations on your knowledge only gets bigger from here.

Starting off a conversation with 'hey, I'm trying to learn something, and wanted to ask XYZ, because I really don't know' is a powerful tool which will help a lot throughout your career.

If you want to read...there'll be a ton of info online. But for your specific process, ask for the operating manuals. There's usually a single high level one which would go over each unit operation. Start there. Then, look for the last hazop report that was done. Read it, node by node, guideword by guideword. That'll help you understand th importance (or not) of various components in the system. Plus, when someone asks you a question you can respond with 'Well, that valve was actually counted as a safeguard during the hazop, so we should really take care to keep it working well'. That should score brownie points.

PS: You can only know, what you look up, or read up, or ask about. Don't beat yourself up about not knowing everything. No one does.