Usually a CTO or an engineering lead, before a reorganisation
Tag each system with its owning team, then roll the architecture up into a team map with every dependency between teams on it, published as a link.
A system's structure mirrors the teams that build it, and the two drift apart quietly. The symptoms are social before they are technical: a change that needs three teams to agree, a service nobody feels responsible for, one person every incident routes through.
The map of systems lives in one place and the org chart in another. Nobody has the picture that puts them together: which team owns what, and which teams depend on which because their systems do.
The same steps the product’s playbook walks. Each one shows the screen at that moment.
The owner:team tag ships with the organisation's context tags. On the landscape, give each system the team that owns it. The playbook's step ticks once most of the systems carry one.

Ask chat to draw the team map: one node per team, and an edge wherever one team's system depends on another's. Every edge between systems owned by different teams becomes a dependency between those teams.

Publish the team map as a link, so the people deciding a reorganisation or a hire read the same picture the engineers work from.

End proofDone when the team map is published as a link, with every dependency between teams on it.
An org chart in one tool and an architecture drawing in another, joined in nobody's head but the lead's.
A code owners file per repository, which says who reviews a folder and nothing about which teams depend on which.
The owning team on each system, rolled up into a map of teams with the dependencies their systems create between them, published as the link a reorganisation is decided with.
Start with one system
Open one repository and run the code plugin. No migration, no rollout, no diagramming.
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Or for writing on architecture that stays connected across systems.