Conway's Law
Systems tend to mirror the communication structure of the organisation that builds them.
Stated in 1967 and never seriously contradicted since: an organisation with four teams will produce a system with roughly four parts, and the interfaces between those parts will look like the interfaces between the teams.
The architectural consequence is that a design that cuts across the organisation chart will be fought by gravity for its entire life. Two teams sharing ownership of one service will drift into a de facto split; one team owning six services will merge them in practice even if not in deployment.
The inverse Conway manoeuvre is the deliberate use of this: change the team structure to the shape you want the architecture to be, then let the architecture follow. It works, slowly, and it requires authority most architects do not have — which is why the more common advice is simply to design with the organisation you actually have.