Question: What is a Universal theme and how does it differ from an FSE theme?
Answer: Universal is a new term for a block based theme that aims to work in either classic (customizer) or FSE contexts.
It’s a new term coined by designers and developers in Automattic. One example would be a theme which uses block template parts in PHP templates.
An alternative to a Universal theme is a “hybrid” theme which uses PHP to override the normal processing of an FSE template.