DH1 = DH(IKa, SPKb)
DH2 = DH(EKa, IKb)
DH3 = DH(EKa, SPKb)
DH4 = DH(EKA, OPKB)
SK = KDF(DH1 || DH2 || DH3 || DH4)
Where IKa is the Alice's identity key, IKb is Bob's Identity key, SPKb is Bob's signed prekey, EKa is Alice's ephemeral key and OPKb is Bob's one-time prekey. All these keys are public.
In the next section they describe how Bob receives this message. Bob loads his private identity key and private one-time pre key and does the same steps as Alice does before to derive the SK. I couldn't understand how this SK calculated by Bob can decrypt the message sent by Alice. Alice uses only public keys and Bob uses his private keys. So my question is: How can the SK generated by Bob decrypt the message sent by Alice?