SQL Server 2017

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

DECLARE @VendorRegistrations TABLE (ID VARCHAR(20), CompanyName VARCHAR(8))
DECLARE @VendorPointOfContacts TABLE (ID VARCHAR(20), VendorRegistrationId  VARCHAR(8), ContactName varchar(100))
DECLARE @VendorKeyDecisionMakers TABLE (ID VARCHAR(20), VendorRegistrationId  VARCHAR(8), DecisionMakerName varchar(100))
DECLARE @VendorLegalRegistrations TABLE (ID VARCHAR(20), VendorRegistrationId  VARCHAR(8), RegistrationNumber varchar(100))
DECLARE @VendorBusinessActivities TABLE (ID VARCHAR(20), VendorRegistrationId  VARCHAR(8), Activity   varchar(100))

INSERT INTO @VendorRegistrations VALUES ('1', 'abc');
INSERT INTO @VendorRegistrations VALUES ('2', 'pqr');
INSERT INTO @VendorRegistrations VALUES ('3', 'def');

INSERT INTO @VendorPointOfContacts VALUES ('1', '1', 'Contact1');
INSERT INTO @VendorPointOfContacts VALUES ('2', '1', 'Contact2');
INSERT INTO @VendorPointOfContacts VALUES ('3', '2', 'Contact11');
INSERT INTO @VendorPointOfContacts VALUES ('4', '3', 'Contact21');

INSERT INTO @VendorKeyDecisionMakers VALUES ('1', '1', 'dec1');
INSERT INTO @VendorKeyDecisionMakers VALUES ('2', '1', 'dec2');
INSERT INTO @VendorKeyDecisionMakers VALUES ('3', '2', 'dec3');
INSERT INTO @VendorKeyDecisionMakers VALUES ('4', '3', 'dec7');

INSERT INTO @VendorLegalRegistrations VALUES ('1', '1', '1');
INSERT INTO @VendorLegalRegistrations VALUES ('2', '1', '2');
INSERT INTO @VendorLegalRegistrations VALUES ('3', '2', '5');
INSERT INTO @VendorLegalRegistrations VALUES ('4', '3', '9');

INSERT INTO @VendorBusinessActivities VALUES ('1', '1', 'painting');
INSERT INTO @VendorBusinessActivities VALUES ('2', '1', 'washing');
INSERT INTO @VendorBusinessActivities VALUES ('3', '2', 'labour');
INSERT INTO @VendorBusinessActivities VALUES ('4', '3', 'architech');

SELECT  A.*, B.ContactName, C.DecisionMakerName, D.RegistrationNumber, E.Activity
FROM    @VendorRegistrations A
INNER JOIN ( SELECT ID, VendorRegistrationId, ContactName, ROW_NUMBER() OVER (ORDER BY ID) Row_ID
             FROM   @VendorPointOfContacts
	) B ON B.VendorRegistrationId = A.ID
INNER JOIN ( SELECT ID, VendorRegistrationId, DecisionMakerName, ROW_NUMBER() OVER (ORDER BY ID) Row_ID
             FROM   @VendorKeyDecisionMakers
	) C ON C.Row_ID = B.Row_ID AND C.VendorRegistrationId = A.ID
INNER JOIN ( SELECT ID, VendorRegistrationId, RegistrationNumber, ROW_NUMBER() OVER (ORDER BY ID) Row_ID
             FROM   @VendorLegalRegistrations
	) D ON B.Row_ID = D.Row_ID AND D.VendorRegistrationId = A.ID
INNER JOIN ( SELECT ID, VendorRegistrationId, Activity, ROW_NUMBER() OVER (ORDER BY ID) Row_ID
             FROM   @VendorBusinessActivities
	) E ON E.Row_ID = B.Row_ID AND E.VendorRegistrationId = A.ID

ID	CompanyName	ContactName	DecisionMakerName	RegistrationNumber	Activity
1	abc	Contact1	dec1	1	painting
1	abc	Contact2	dec2	2	washing
2	pqr	Contact11	dec3	5	labour
3	def	Contact21	dec7	9	architech