How much memory does the RS826RP+ come with, and what is its maximum memory capacity?

It comes with 4GB DDR4 installed memory and supports up to 32 GB total using 2 DDR4 SODIMM slots.

What type of drives can be installed in this Synology RackStation RS826RP+?

It supports 4 x 3.5-inch SATA HDDs or 2.5-inch SATA SSDs across its 4 drive bays.

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Synology 4bay NAS RackStation - AMD V1500B (2.2 GHz) - 2 Ram Slots - 4 Drive Bays - Gigabit Ethernet - USB 3.2 - 4x RJ-45 - RS826RP+