When to use {{}} ? How to interpolate variables or...

A steadfast rule is ‘always use {{ }} except when when:‘. Conditionals are always run through Jinja2 as to resolve the expression, so when: failed_when: and changed_when: are always templates and you should avoid adding {{}}.
It is always recommended to use brackets even if you have previously used variables without specifying (like with_clauses). This makes it hard to distinguish between a string and an undefined variable.
Another rule is ‘moustaches don’t stack’. We often see this:
{{ somevar_{{other_var}} }}
The above DOES NOT WORK, if you need to use a dynamic variable use the hostvars or vars dictionary as appropriate:
{{ hostvars[inventory_hostname][‘somevar_’ + other_var] }}

When to use {{}} ? How to interpolate variables or dynamic variable names?