DDR4 pull-up resistors and decoupling clock lines
Because of the insanely fast speeds they run at, DDR4 interfaces are designed as impedance controlled transmission lines.  The resistors are pulled to a voltage with a "VTT" on it, indicating that it is the termination voltage.  The clock lines are differential AC terminated to 1.2V.  You must understand transmission lines and termination and be prepared to design an impedance controlled PCB if you want to create a functioning DDR4 interface.

You can Google lots of information on DDR4 design, for example this link from EDN and this link from NXP.