but where are this log files kept
Where ever you choose to put the log backups that you take. Mirroring doesn't use log backups, so where they go is irrelevant. I imagine you'll put them with the rest of your backups
Second how this log are sent i mean using sql agent or through any other way
They're sent automatically across the mirroring end points that you set up when you configured mirroring.
May I suggest google and look for a good article on the basics of mirroring?
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