How to put all the data in one column of second table in internally joined in one column of first table

There's many ways to achieve that...

1) using CTE[^].
Before i show you how to use it, i need to mention that your database structure is wrong. You need a "in-between-table" which will link subjects with students. See:

DECLARE @tempStudent TABLE(	Student_Id int, [Name] nvarchar(10), Class nvarchar(10))
Insert Into @tempStudent Values(1,'Joy', 'C1'),(2,'Root', 'C2'),(3,'Devil', 'C3')
DECLARE @tempSubject TABLE( Subject_Id int, SubjectName nvarchar(10))
Insert Into @tempSubject Values(1,'English'),(2,'Math'),(3,'Science')
--new table with relationship many (Student_id) to many (Subject_id)
DECLARE @tempSubjectStudent TABLE( Student_Id int, Subject_Id int)
Insert Into @tempSubjectStudent Values(1, 1),(1,2),(2,3),(2,2),(3,3),(3,1)

Now, an example CTE query:

;WITH CTE AS
(
	--initial query
	SELECT DISTINCT st.Student_Id, st.[Name], st.Class, CONVERT(NVARCHAR(MAX), '') AS SubjectName, 
		0 AS RowNo, 0 AS Subject_Id , COUNT(*) OVER(PARTITION BY st.Student_Id) AS NoOfSubjects
	FROM @tempStudent st INNER JOIN @tempSubjectStudent ss ON st.Student_Id = ss.Student_Id 
	--recursive part
	UNION ALL
	SELECT st.Student_Id, st.[Name], st.Class, CONCAT(st.SubjectName, su.SubjectName, ', ') AS SubjectName,
		st.RowNo + 1 AS RowNo, ss.Subject_Id AS Subject_Id ,st.NoOfSubjects
	FROM CTE st
		INNER JOIN @tempSubjectStudent ss ON st.Student_Id = ss.Student_Id  
		INNER JOIN @tempSubject su ON ss.Subject_Id = su.Subject_Id
	WHERE st.RowNo<= st.NoOfSubjects AND ss.Subject_Id > st.Subject_Id
)
SELECT  [Name], Class, SubjectName
FROM CTE 
WHERE RowNo=NoOfSubjects

Result:

Name	Class	SubjectName
Devil	C3	English, Science, 
Root	C2	Math, Science, 
Joy		C1	English, Math, 

For further details, please see:
WITH common_table_expression (Transact-SQL) - SQL Server | Microsoft Docs[^]
Common Table Expressions (Introduction to CTE's) - Essential SQL[^]
Using a Recursive CTE to Generate a List – SQLServerCentral[^]

2) using FOR XML [^]
See:
How to concatenate text from multiple rows into a single text string in SQL server? - Stack Overflow[^]
Examples: Using PATH Mode - SQL Server | Microsoft Docs[^]
How Stuff and 'For Xml Path' work in Sql Server - Stack Overflow[^]