The PostgreSQL Select Syntax

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/16/sql-select.html
Description:

Dear PostgreSQL Team,

I'm writing to seek clarification regarding the scope of the formal syntax
outlined in the documentation for the SELECT statement
(https://www.postgresql.org/account/comments/new/16/sql-select.html/)

I understand that this syntax defines the core structure and rules for
constructing valid queries. However, I'm interested in building an
Object-Relational Mapper (ORM) for PostgreSQL.

My primary question is: Does the documented formal syntax represent the
absolute limit of what PostgreSQL can understand and process, or are there
additional functionalities beyond these core rules? I'm particularly
interested in how ORMs typically handle features like data type mapping,
object-relational mapping, and support for advanced functionalities like
subqueries and window functions.

I'm asking this for clarity to avoid dedicating time to exploring the
internal C parser code of PostgreSQL. I'd like to ensure the documented
syntax serves as a reliable foundation for building an ORM, with the
understanding that there might be additional functionalities beyond the core
structure.

Any insights you can provide on this topic would be greatly appreciated.
Additionally, if there are resources beyond the formal syntax documentation
that would be helpful for building a PostgreSQL ORM, I'd be grateful if you
could point me in the right direction.

Thank you for your time and assistance.