I want insert trigger to be fired in another table when the condition satisfies

CREATE TRIGGER TRI_INSERTINSTOCK 
   ON  instock 
   FOR INSERT
AS
BEGIN
	SET NOCOUNT ON;

	DECLARE @storeid int    
	SELECT @storeid=storeid FROM inserted
    
DECLARE @Is_instock bit
SET @Is_instock =0
SELECT @Is_instock = 1 FROM instock WHERE STOREID =@storeid 
	
DECLARE @Is_bigStores bit
SET @Is_bigStores=0
SELECT @Is_bigStores= 1 FROM bigstores WHERE STOREID =@storeid 

	IF (@Is_instock=1 AND  @Is_bigStores=0)
	insert into bigstores(storeid,city,phone)
	select storeid,city,phone
	from stores
	where storeid in(select storeid
	from instock 
	group by storeid
	HAVING sum(QUANTITY) >=50000 AND storeid=@storeid
	)

END

--create tables
CREATE TABLE bigstores(storeid INT,city NVARCHAR(50),phone NVARCHAR(50))
CREATE TABLE stores (storeid INT,city NVARCHAR(50),phone NVARCHAR(50))
CREATE TABLE instock(storeid INT,QUANTITY INT)

--Insert date into instock
INSERT INTO instock
SELECT 1,10000
UNION ALL
SELECT 1,20000
UNION ALL
SELECT 2,40000
UNION ALL
SELECT 3,10000
UNION ALL
SELECT 3,40000

--Insert data to stores
INSERT INTO stores
SELECT 1,'CITY1','+111251252'
UNION ALL
SELECT 2,'CITY1','+111251285'
UNION ALL
SELECT 3,'CITY1','+111255455'

--Run query to creat trigger

--Run query to test trigger
INSERT INTO instock 
SELECT 3,20000
SELECT * FROM bigstores