Why miners looking for zeros?
Is it true that miners are looking for a number of zeros ...
... because if they needed to find the complete hash ...
It is nothing to do with partial or complete hashes. All hashes computed by miners are complete hashes of the hashed data.
... they will be able to see the content of the transaction?
No, miners don't need a hash in order to see the contents of the transactions.
Besides that, it is probably impossible to find a complete hash?
No. it is easy to compute a complete hash. A single modern hardware device can compute 100,000,000,000,000 complete hashes every second.
A hash is an unpredictable number between 0 and roughly 11579208923731619542357098500869123456789012345678901234567890123456789
Therefore most hashes will be very very big numbers and so it is difficult to find a set of data whose hash is in some small part of that range (e.g. within 0 to 21)
Miners are using a brute-force trial-and-error method of finding a set of data whose hash is numerically less than the current network target value.
For example, if the network target was 21, then a hash of 33 would be too big but a hash of 17 would be a winner. In Bitcoin the target numbers are currently much larger than 21 but the comparison is still numeric.
To find a set of data that hashes to a value smaller than the target they vary a part of the block data which is known as the nonce. For each variation they compute the hash and see if the hash is less than the target. There are other things the miners can change that also affect the value of the computed hash. Miners have to resort to changing these other things after they have tried every possible value of nonce.
The target is set by Bitcoin rules such that a hash less than the target will, on average, be found every ten minutes.
This is a lot of work. The arrangement of data whose hash is less than the target is proof of this work having been done. Hence the phrase proof-of-work.