When should data in YAML be a list, and when an object (mapping)? Why is a playbook a list of plays, but the arguments of a module an object?

Use a list for an open-ended sequence of similar items: order matters, duplicates are allowed, and you reach items by position. Use an object to describe one thing by its named properties: each key is unique, key order doesn't matter, and you reach values by key.

* Plays and tasks are lists because more can follow in order; module arguments are one object. *

users:            # list: more users can be appended, "Bob" may repeat
  - Alice
  - Bob
user:             # object: the properties of one user
  name: Alice
  age: 30
  active: true

A playbook follows the same reasoning at every level:

• The playbook is a list of plays, because you can always add another play ("now configure the database servers"), and plays run in the order written.
• tasks: is a list of tasks for the same reason: an extendable sequence of steps that run one after another.
• The arguments of a module are an object. ansible.builtin.apt: {name: apache2, state: present} lists properties of a single desired state. Each argument appears once, and swapping name and state changes nothing.

A quick test: ask "could there be another one, and does the order matter?" If yes, it's a list. If you are setting named attributes of a single thing, it's an object.

Go deeper:

• YAML Syntax: YAML basics (official docs) — lists and dictionaries, and how playbooks nest them.
• YAML 1.2.2 specification: sequences and mappings — the spec's own examples of sequences, mappings and their nesting.
• YAML: syntax (Wikipedia) — sequences and mappings, block and inline style.