What does RRQMS and WRQMS mean?
RDS instances use Elastic Block Storage (EBS). EBS documentation explains what request merge means:

the underlying drive technology determines the maximum amount of data that a volume type counts as a single I/O. I/O size is capped at 256 KiB for SSD volumes and 1,024 KiB for HDD volumes because SSD volumes handle small or random I/O much more efficiently than HDD volumes.
When small I/O operations are physically contiguous, Amazon EBS attempts to merge them into a single I/O operation up to the maximum size. For example, for SSD volumes, a single 1,024 KiB I/O operation counts as 4 operations (1,024÷256=4), while 8 contiguous I/O operations at 32 KiB each count as 1 operation (8×32=256).

High number of merged I/O requests means your database is requesting a lot of sequential blocks, and EBS is able to translate that into fewer but larger, hence more efficient I/O operations. Low number of merged I/O requests probably means your database is mostly accessing random blocks.
To answer your

Came from where? Issued by whom?

-- by whatever is reading from and writing to EBS volume(s) on your RDS instance, presumably Postgres.