How is each secure development phase described, and who is accountable in every one of them?

Every phase has a goal, deliverables, prerequisites, a RACI line, five steps, and dos and don'ts. The Product Owner is Accountable in all ten.

RACI splits ownership of a task four ways:

• Responsible does the work.
• Accountable owns the outcome and signs off. There is exactly one.
• Consulted gives input before the work is done (two-way).
• Informed is told the result (one-way).

The deck adds Supporting for roles that help with the work. Making the Product Owner accountable everywhere is deliberate. Security then becomes a property of the product, owned by the person who owns the product, and not a side task of the security team. The Responsible role moves from phase to phase:

Go deeper:

• Responsibility assignment matrix (Wikipedia) — RACI and its variants, with the one-accountable rule