Yes, Nokia 5.1 does have the fingerprint sensor. It is located on the back panel of the phone.
Nokia 5.1 has a 3GB LPDDR3 type RAM. It is a single channel.
No. Nokia 5.1 has 2 dedicated slots for the sim cards and 1 dedicated slot for the MicroSD card. You can use up to a 128GB memory card in Nokia 5.1.
No. Nokia 5.1 does not have the face unlock.