How do I design a trigger for Postgresql, and what is wrong with my "Hello world" test?

I need to use triggers in postgresql (preferably via SQLALchemy ORM). However, I can’t seem to get to first base with creating one.
As a basic test, I am trying to put the string "Hello world" into a column called test each time a row is inserted.

I set up my tables like this:

from __future__ import annotations
from typing import List
import sqlalchemy
from sqlalchemy import create_engine, ForeignKey, Column, Integer, String, CheckConstraint, MetaData
from sqlalchemy.orm import DeclarativeBase, Mapped, DeclarativeBase, MappedAsDataclass, mapped_column, relationship
from sqlalchemy.schema import DDL
import datetime

dbUrl = 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX'

engine = create_engine(dbUrl,
                       echo=True)

metadata = MetaData()



class Base(MappedAsDataclass, DeclarativeBase):
  pass

class Company(Base):
    __tablename__ = "company"
    id: Mapped[int] = mapped_column(primary_key=True)
    name: Mapped[str]
    managers: Mapped[List[Manager]] = relationship(back_populates="company")

Company.__table__
Base.metadata.create_all(engine)

class Employee(Base):
    __tablename__ = "employee"
    id: Mapped[int] = mapped_column(primary_key=True)
    name: Mapped[str]
    type: Mapped[str]
    test: Mapped[str] = mapped_column(nullable=True)

    __mapper_args__ = {
        "polymorphic_identity": "employee",
        "polymorphic_on": "type",
    }

Employee.__table__
Base.metadata.create_all(engine)


class Manager(Employee):
    __tablename__ = "manager"
    id: Mapped[int] = mapped_column(ForeignKey("employee.id"), primary_key=True)
    manager_name: Mapped[str]
    CheckConstraint("manager_name == employee.name", name="check1")

    company_id: Mapped[int] = mapped_column(ForeignKey("company.id"))
    company: Mapped[Company] = relationship(back_populates="managers")

    __mapper_args__ = {
        "polymorphic_identity": "manager",
    }

class Engineer(Employee):
    __tablename__ = "engineer"
    id: Mapped[int] = mapped_column(ForeignKey("employee.id"), primary_key=True)
    engineer_name: Mapped[str]

    company_id: Mapped[int] = mapped_column(ForeignKey("company.id"))
    company: Mapped[Company] = relationship(back_populates="engineers")

    __mapper_args__ = {
        "polymorphic_identity": "engineer",
    }  
Engineer.__table__
Base.metadata.create_all(engine)

and then tried to creat a trigger like this:

sqlalchemy.event.listen(metadata,
                       "before_create",
                       DDL("""CREATE TRIGGER autoCreateEmployee
                       AFTER INSERT
                       ON employee
                       FOR EACH ROW
                       INSERT INTO employee(test)
                       VALUES("Hello world"),
                       0)  
                       """))

The tables were created and no error was emitted, but as far as I can tell, the trigger was not added. I also tried running my test trigger statement directly in SQL using DBeaver:

CREATE TRIGGER autoCreateEmployee
                       AFTER INSERT
                       ON employee
                       FOR EACH ROW
                       INSERT INTO employee(test)
                       VALUES("Hello world")

That gave me this error:

SQL Error [42601]: ERROR: syntax error at or near "INSERT"
  Position: 165

Error position: line: 5 pos: 164

What is wrong with my trigger statement? And why don’t I see the error when running via SQLAlchemy?

Edit: I was attempting to adapt this example:

Update: Based on the comments, I’ve taken another look at the docs and realized that some of the examples I were looking at weren’t even Postgres. Now I can run the commands to create a function and trigger without error, but as far as I can tell, the trigger exists (as I cannot re-create it without deleting), but doesn’t appear to be associated with the table it’s supposed to be on.

At the moment I am just running raw SQL to avoid ORM complications:

drop function hello_world;

CREATE FUNCTION hello_world() RETURNS trigger AS $hello_world$
    BEGIN
        INSERT INTO employee(test) VALUES ("HELLO WORLD!");
        RETURN NEW;
    END;
$hello_world$ LANGUAGE plpgsql;

drop trigger hello on employee;

CREATE TRIGGER hello
    AFTER UPDATE ON employee
    FOR EACH ROW
    EXECUTE FUNCTION hello_world();

Edit2: Deleted a screenshot because it was the wrong screenshot and based on a misunderstanding anyway.

Current status:

My function is like this:

CREATE FUNCTION hello_world()
RETURNS trigger
LANGUAGE PLPGSQL
AS $hello_world$
    BEGIN
        INSERT INTO employee(test) VALUES ('HELLO WORLD!');
    END;
$hello_world$

and I create my trigger like this:

CREATE TRIGGER hello
    AFTER INSERT ON employee
    FOR EACH ROW
    EXECUTE FUNCTION hello_world();

However, it tries to create a NEW row with just the test string in the test column instead of into the row on which I just entered data. I was under the impression that the FOR EACH ROW meant that the function will be carried out on all rows being inserted. Obviously I am still missing something.

If I run:

INSERT INTO employee (name,type) VALUES ('Merv', 'manager');

I get

SQL Error [23502]: ERROR: null value in column "name" of relation "employee" violates not-null constraint
  Detail: Failing row contains (31, null, null, HELLO WORLD!).
  Where: SQL statement "INSERT INTO employee(test) VALUES ('HELLO WORLD!')"
PL/pgSQL function hello_world() line 3 at SQL statement

So now I need to figure out how to target the just-modified row(s) with the trigger function.