What does t and u mean in presto sql

I am reading the documentation and trying to get to the bottom of some queries.

Here is a query with both t and u

SELECT * FROM (VALUES 1, 2) t("left"), (VALUES 3, 4) u("right");

Here is another using only t:

SELECT * FROM (VALUES 5, 2, 4, 1, 3) t(x) ORDER BY x OFFSET 2 LIMIT 2;

There are also these two (notice the space between the t and the ():

SELECT student, score
FROM tests
CROSS JOIN UNNEST(scores) AS t (score);

FROM (
  VALUES
    (ARRAY[2, 5], ARRAY['dog', 'cat', 'bird']),
    (ARRAY[7, 8, 9], ARRAY['cow', 'pig'])
) AS x (numbers, animals)
CROSS JOIN UNNEST(numbers, animals) AS t (n, a);

Here t(sites) is described as an alias.

FROM dataset, UNNEST(items) AS t(sites)

I have googled for hours. I hope someone with more SQL experience can point me toward useful search strings.

Are both u and t predefined aliases?
What do the aliases mean
Do I need the parentheses for the alias to work?
Do the space between the character and the parentheses have any special meaning?

Solution

My DWH team helped me out. These are aliases combined with column names. When you know, it seems trivial.

The first example:

SELECT * from (VALUES 1, 2) t("x")

might be easier to understand written like this

SELECT * from (VALUES 1, 2) as t("first")

The (VALUES 1, 2) as t("first") creates a table with one column and two rows. The table has the alias t, and column name first.

SELECT * from (VALUES (1,3), (2, 4)) as t("first", "second")

Gives two rows like this

#    first  second
1    1      3
2    2      4

They are not predefined aliases but normal aliases
NA
You need them if the column names can not be inferred from the table
No