Services, jobs, scheduled tasks, external dependencies and the things nobody remembers deploying.
How the parts relate, where the data lives, what talks to what, and which of those connections are load-bearing.
Where it runs, how a change reaches production, and what happens today when something goes wrong.
The parts that would hurt most if they failed, named before they fail rather than afterwards.
Pick the level of involvement that fits, describe your product, and we begin there.