`timestamptz` doesn't convert to UTC, it has no timezone - or rather, it gets interpreted as having whatever TZ the session is set to, which could change anytime. Postgres stores the value as a 64 bit microseconds-since-epoch. `timestamp` is the same. It's sad that even the official PG docs get this wrong, and it causes problems downstream like the JDBC driver natively mapping it to OffsetDateTime instead of Instant.
But you're right that timestamptz on postgres is different from timestamptz on oracle, which _does_ store a timezone field.
But you're right that timestamptz on postgres is different from timestamptz on oracle, which _does_ store a timezone field.