Another SQL tool you could use is MERGE
--Create first table
CREATE TABLE #aa(
    username varchar(50),
    userid varchar(50),
    userfullname varchar(50),
    useremail varchar(50)
)
GO
--Create second table
CREATE TABLE #bb(
    username varchar(50),
    userid varchar(50),
    userfullname varchar(50),
    useremail varchar(50)
)
GO
--Insert all data into temp table #aa
INSERT #aa (username,userid,userfullname,useremail)
VALUES
    ('AAA','ID1','JOHN DOE','AAA@YAHOO.COM'),
    ('BBB','ID2','BILLY BOB','BBB@YAHOO.COM'),
    ('CCC','ID3','TOM HANKS','CCC@GMAIL.COM'),
    ('DDD','ID4','CONNOR MCGREGOR','DDD@GMAIL.COM'),
    ('EEE','ID5','FLOYD MAYWEATHER','EEE@GMAIL.COM')
GO
--Insert only the username in temp table #bb
INSERT #bb (username)
VALUES
    ('AAA'),
    ('BBB'),
    ('CCC'),
    ('DDD'),
    ('EEE')
GO
--See #bb before
SELECT * FROM #bb
MERGE #bb AS TARGET
USING #aa AS SOURCE
ON TARGET.username = SOURCE.username
WHEN MATCHED
THEN UPDATE SET
    TARGET.userid = SOURCE.userid,
    TARGET.userfullname = SOURCE.userfullname,
    TARGET.useremail = SOURCE.useremail;
GO
--See #bb after
SELECT * FROM #bb
DROP TABLE #aa
DROP TABLE #bb
Before Merge
+----------+--------+--------------+-----------+
| username | userid | userfullname | useremail |
+----------+--------+--------------+-----------+
|  AAA     |  NULL  |    NULL      |   NULL    |
|  BBB     |  NULL  |    NULL      |   NULL    |
|  CCC     |  NULL  |    NULL      |   NULL    |
|  DDD     |  NULL  |    NULL      |   NULL    |
|  EEE     |  NULL  |    NULL      |   NULL    |
+----------+--------+--------------+-----------+
After Merge
+----------+--------+-------------------+----------------+
| username | userid |  userfullname     | useremail      |
+----------+--------+-------------------+----------------+
|  AAA     |  ID1   |  JOHN DOE         | AAA@YAHOO.COM  |
|  BBB     |  ID2   |  BILLY BOB        | BBB@YAHOO.COM  |
|  CCC     |  ID3   |  TOM HANKS        | CCC@GMAIL.COM  |
|  DDD     |  ID4   |  CONNOR MCGREGOR  | DDD@GMAIL.COM  |
|  EEE     |  ID5   |  FLOYD MAYWEATHER | EEE@GMAIL.COM  |
+----------+--------+-------------------+----------------+