How can I join msdb.dbo.sysalerts to msdb.dbo.sysoperators?
You can use this query:

SELECT * 
FROM msdb.dbo.sysalerts AS Ale
INNER JOIN msdb.dbo.sysnotifications AS Noti ON Ale.id = Noti.alert_id
INNER JOIN msdb.dbo.sysoperators AS Ope ON Noti.operator_id = ope.id