How UPDATE(column) function works in case of batch updates?

SQL Server triggers execute on batches instead of each row in the update transaction.

If 'N' rows of a table were updated in a single transaction, the UPDATE Trigger will execute on the 'N' rows together. (UPDATE(): Link to MS Docs)

How does the trigger function UPDATE() work in such scenario, assuming the following table:

CREATE TABLE [dbo].[Sample](
    SampleId INT IDENTITY(1,1) NOT NULL,
    Column1 VARCHAR(100) NULL,
    CONSTRAINT [PK_Sample] PRIMARY KEY CLUSTERED (SampleId ASC)
);

Considering this update on the table:

UPDATE Sample SET Column1 = 'Example' WHERE SampleId <= 5;

And, if inside the update trigger for above table there is this condition:

IF UPDATE(Column1)

• Will the above condition evaluate to True if Column1 is updated in each of the 'N' updates, or will it evaluate to True if any one of the 'N' updates have an update on Column1?
• Or is there a way to use it on each update one by one?

Edit: <br>
If there are 2 different updates on the same table in a single transaction, will the Update trigger fire once for each Update statement, or once for all updates in that transaction?

If the column is included in the SETclause of the UPDATE then UPDATE() returns true; it's as simple as that. UPDATE() considers the statement not the row or the data.

Take a simple statement like this:

UPDATE SomeTable
SET SomeColumn = 1
WHERE SomeColumn = 1

This would result in UPDATE(SomeColumn) being true; SomeColumn was included in the SET clause. UPDATE(AnotherColumn) would be false, as it wasn't included.

If there are multiple rows effected, or none, it doesn't matter.

As a full working example:

CREATE TABLE dbo.SomeTable (ID int IDENTITY(1,1),
                            SomeInt int,
                            SomeString varchar(10));
GO

INSERT INTO dbo.SomeTable (SomeInt,SomeString)
VALUES(1,'abc'),
      (2,'def'),
      (2,'ghi');
GO

CREATE TRIGGER trg_SomeTrigger ON dbo.SomeTable
AFTER UPDATE AS
BEGIN

    IF UPDATE(SomeString)
        PRINT N'SomeString was updated';
END;
GO
--Prints statement, SomeString was updated.
--1 row is affected, though the value doesn't change.
UPDATE dbo.SomeTable
SET SomeString = 'abc'
WHERE SomeInt = 1;
GO
--Prints statement, SomeString was updated.
--2 rows are affected, the values do change.
UPDATE dbo.SomeTable
SET SomeString = '123'
WHERE SomeInt = 2;
GO
--Prints statement, SomeString was updated.
--0 rows are affected, no values change.
UPDATE dbo.SomeTable
SET SomeString = 'abc'
WHERE SomeInt = 3;
GO
--Doesn't Print statement, as SomeString wasn't updated.
--1 row is affected, but not SomeString.
UPDATE dbo.SomeTable
SET SomeInt = 3
WHERE SomeInt = 1;
GO

DROP TABLE dbo.SomeTable;

If you want to check if a row and column's value has changed you need to inspect the values in the inserted and deleted pseudo tables. If you are on SQL Server 2022 then this is much easier with IS DISTINCT FROM. On older versions, if the column is NULLable you'll need to ensure you check for that too. For example:

CREATE TRIGGER trg_AnotherTrigger ON dbo.SomeTable
AFTER UPDATE AS
BEGIN

    IF EXISTS (SELECT 1
               FROM inserted i
                    JOIN deleted d ON i.ID = d.ID
               WHERE i.SomeString IS DISTINCT FROM d.SomeString)
        PRINT N'SomeString was updated and a value was changed';
END;

The above trigger would only PRINT for the second UPDATE statement, where values of SomeString where changed.
db<>fiddle