ansible variables usage - ansible_hostname or {{ ansible_hostname }} - ansible

Question

I am new to ansible. What is the correct to call ansible variables? Here are the 3 playbooks, playbook 1 uses "{{ ansible_hostname }}", however, playbook 2 and 3 uses "ansible_hostname" directly. What are the differences? Thanks!
Playbook 1:
tasks:
- name: Jinja2 template
template:
src: template.j2
dest: "/tmp/{{ ansible_hostname }}_template.out"
trim_blocks: true
mode: 0644
Playbook 2:
tasks:
- name: Ansible Jinja2 if
debug:
msg: >
--== Ansible Jinja2 if statement ==--
{# If the hostname is ubuntu-c, include a message -#}
{% if ansible_hostname == "ubuntu-c" -%}
This is ubuntu-c
{% endif %}
Playbook 3:
tasks:
- name: Exploring register
command: hostname -s
when:
- ansible_distribution == "CentOS"
- ansible_distribution_major_version | int >= 8
register: command_register

Answer 1

playbook 1 uses "{{ ansible_hostname }}", however, playbook 2 uses "ansible_hostname"
That's not entirely correct. Both playbooks use the variable name ansible_hostname inside a Jinja templating context.
In the first playbook, it's simple variable substitution, so we use the {{ ... }} markers.
In the second playbook, it's being used in a control expression, so we use the {% ... %} markers.
In the third playbook, you're looking at the clauses of a when expression. From the documentation:
The when clause is a raw Jinja2 expression without double curly braces...
You can read more about Jinja syntax here.