I have delivered systems two ways. Some I keep operating. Some are executed and run by somebody else, sometimes by a different company entirely. The second kind is harder, and not for the reason people assume.
It is not a documentation volume problem. You can write forty pages and still hand over something nobody can maintain.
The part that does not survive is the judgment
Code explains itself reasonably well. A gate does not. Somewhere in every pipeline I have built there is a threshold set to a specific number, a filter that excludes a specific category, an abbreviation list that looks arbitrary until you know which forms actually appear in the source data.
Every one of those encodes a decision, usually one made after being burned. Ship the code without the decision and the next engineer reads it as noise. Then they clean it up, and the bug you fixed in March comes back in September wearing a different hat.
A spec is usable when its requirements carry reasons
On one engagement the deliverable was not an implementation at all. It was an architecture spec that the client's existing development partner would build against, followed by months of my reviewing their work against it.
What made that spec work was not completeness. It was that each non-obvious requirement said why. Not why in the abstract, but which failure it prevented. A requirement with a reason attached survives contact with a developer who disagrees with it, because now the disagreement is about the reason rather than about taste.
Observability is part of the deliverable
If someone else is going to run it, they need to be able to tell that it is broken without asking me. That means per-run cost, per-stage status, error classification that separates a lookup finding nothing from a lookup throwing, and a way to replay a bad window rather than reconstruct it from logs.
I used to treat that as maturity work for later. It belongs in the first delivery. A system nobody can inspect is a system that only I can operate, and if that is what I hand over then I have not handed anything over.
The uncomfortable part
A system you no longer operate will drift. Someone will loosen a filter to hit a number this quarter. Someone will add a manual step and forget to write it down. Six months later it does not behave the way you built it.
You do not get to be annoyed about that. It stopped being yours. The only honest response is to have made the drift visible while you were still there, so the people who own it now can see what changed and decide whether they meant it.
The deliverable is never just the pipeline. It is the pipeline plus the reasons it is shaped the way it is.