How does PostgreSQL store timestamp internally? In case of MySQL,

TIMESTAMP values are stored as the number of seconds since the epoch ('1970-01-01 00:00:00' UTC)`

In case of PostgreSQL with the version less than or equal to 9.6

timestamp values are stored as seconds before or after midnight 2000-01-01

In case of PostgreSQL with the version greater than or equal to 10, there is no explanation about this
I have two questions about the internal logic of PostgreSQL.

Does it still use the same standard as the version 9.6?
Why &quot;midnight 2000-01-01&quot;? Unix epoch starts from 1970-01-01 00:00:00 UTC. J2000 epoch starts from 12 noon (midday) on January 1, 2000.

It seems like only a few systems use 2000-01-01 00:00:00.
Because PostgreSQL provides functions to convert UNIX epoch into the timestamp to_timestamp or vice versa EXTRACT(EPOCH FROM ...), using the standard that is different from UNIX epoch seems like to require additional offset calculations.
That information from the 9.6 documentation is wrong. Timestamps are stored as microseconds since 2000-01-01 00:00:00.
See include/datatype/timestamp.h:
/*
 * Timestamp represents absolute time.
 *
 * [...]
 *
 * Timestamps, as well as the h/m/s fields of intervals, are stored as
 * int64 values with units of microseconds.  (Once upon a time they were
 * double values with units of seconds.)
 *
 * [...]
 */

typedef int64 Timestamp;
typedef int64 TimestampTz;

Back in 9.6, there was also the option to store timestamps as floating point values, but that has been removed since.
That should answer your first question.
The reason why 2000 was picked probably stems from the time when you could store timestamps as floating point values. Here, the precision got better for values close to the epoch, and it made sense to pick something more future-proof than 1970 (mind you, that was before 2000).
Subtracting a constant in order to calculate the Unix epoch is no great effort.