pilots - pre fligth check - what do they check?

help me understand this..
before pilots takes off a commerical flight..what are the all various technical checks they do on the airplane..
since it being a big one,how do they check that each and every part is working fine in that short span of time.. i am sure even a small malfunctioning on any
small part can bring the plane down..but,will the pilots or engineers really have time to check each and every single bolt and nuts in the airplane..
 
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