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I am by no means an expert, but if AAD Connect is not used, you can use ADFS. However I not sure that ADFS provide pass-through auth. Also keep in mind that MS is recommending AAD Connect for hybrid identities.

Passthrough isn't mandatory, but it makes things easier as less pre-reqs required. I have just been configuring this recently for a client using AAD Connect with password hash synch & no ADFS. These are my notes for the most seamless experience. Not sure it is possible without AAD connect or ADFS. Hash synch requires you to confirm a few more pre-reqs than are required for pass through (see linked MS page in the notes).