Sql-server – Creating SQL Server Job on each replica in availability group

just need to enable it on the current replica.

You will need to modify you sql agent jobs to include below step

CREATE FUNCTION dbo.fn_hadr_group_is_primary (@AGName sysname)
RETURNS bit 
AS
BEGIN 
DECLARE @PrimaryReplica sysname; 
SELECT @PrimaryReplica = hags.primary_replica 
FROM 
sys.dm_hadr_availability_group_states hags
INNER JOIN sys.availability_groups ag ON ag.group_id = hags.group_id
WHERE
ag.name = @AGName;

IF UPPER(@PrimaryReplica) =  UPPER(@@SERVERNAME)
RETURN 1; -- primary

RETURN 0; -- not primary
  END; 

GO

So you have to create the above function in all your replicas and add as first step in sql agent jobs you wish to execute

If the replica is not primary,you can modify the job to Quit

This has been covered in depth here :
AlwaysOn Availability Groups and SQL Server Jobs, Part 7: Detecting Primary Replica Ownership

The database replica to be taken offline can be primary or secondary.

The database taken offline should always be the secondary.

The database replica has to be take offline and online via transact SQL

That's possible.

When the database replica is offline it should not be possible for it to execute requests. All existing connections must be moved to the other database replica or fail, so that the client can reconnect using the IP listener to the other replica.

Basically here's how it would go:

1. If the database replica that needs to be taken offline is a primary, check for the current secondary to be synchronized.
2. Set the database to be "taken down" as a non-readable secondary.
3. If the database replica was a primary, failover.
4. Do any other items you'd like here, such as removing it from the AG, etc.