The easy version:
HDD is relatively far away from the CPU, connected to system logic board via a somewhat slow SATA connection. SATA 6GB/s is the current standard. HDD's can only read or write but not both at the same time.
RAM sits very close to the CPU and has a very high bandwidth connection. DDR4 has a throughput of ~40GB/s some graphs RAM's can read/read, read/write and write/write at the same time (dual channel). RAM's can also send very large data at once (double data).
The technical version:
HDD's have spinning platters, and a magnetic read/write head needs to reposition every time you want to read/write something. It needs to find the correct places on each platter. HDD's platters are spinning at 5400 rpm, 7200 rpm or 10000 rpm. The magnetic read/write head sits on each side of a platter. Typically an HDD has three platter and six read/write heads. But those heads are fixed to the movement of just one torque engine. Depending on the layout of a filesystem the read/write head can't constantly read from near positions.
RAM memory typically modules consists 4 or 8 memory modules. Each module is connected via dual-channel and read/read, read/write or write/write at the same time. At best a RAM memory with eight modules can 16 times read data at the same time or write 16 times at the same time. Each memory module has its layout manager; that knows exactly where the data is. Reading data is only affected by something called CAS timing. For example a DDR4 RAM clocks at 1800 Mhz (1,800 billion clock cycles / second ) and has a CAS timing of 34; means that the RAM module just needs 34 clock cycles to be ready.
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