Does postgres automatically generate an id for every row?

I am new to postgres and SQL in general. I am coming from NoSQL. More specific mongodb. In mongodb every document had a unique id automatically generated by mongodb.

Do postgres rows automatically have a unique id or do you have to generate them yourself?

Answer

You need to define one for yourself:

create table foo
(
  id integer generated always as identity primary key,
  .... other columns
);

In a relational database you have to define the primary key yourself, the database does not make assumptions about that. e.g. a link table for a many-to-many relationship usually doesn’t have a generated PK at all:

create table many_to_many
(
  one_id integer not null references table_one,
  other_id integer not null references other_table,
  primary key (one_id, other_id)
);