What do I write down before going on holiday?
The three things that break, how to tell which one it is, and who to call when it is none of them. Architecture diagrams are pleasant; runbooks are what get used at two in the morning.
How do I transfer ownership of a service properly?
Have the new owner run the next two deploys and the next incident while you watch. Documents transfer facts, not judgement, and judgement is most of what ownership is.
What should a new hire do in week one?
Ship something tiny on day one, then read. The order matters: a merged one-line fix teaches the whole pipeline, and the codebase reads differently once you have touched it. A week of reading first produces a person who knows the architecture and cannot deploy.
Does this still hold if I am working alone?
It gets easier, not harder. A fair share of the difficulty is coordination, and that disappears. What remains is remembering your own decisions, which is what writing them down is for.