Replicants - those of the Nexus 6 kind, anyway - are artificially created through genetic engineering. They're not mechanical robots, but rather more like clones. It is not entirely clear how they are created as this is not specifically explained in the film. It is also possible that previous Nexus series replicants were more robotic in the 'classic' sense; this is pure speculation, however, since no information is really given about them. What we do know about the Nexus 6 series is that they are engineered by genetic engineers, they are created from some sort of DNA and they are designed to be "More human than human." The likelihood is that this is thus some sort of cloning/replication technology that uses human DNA as a starting point..

The 1981 script also provided the following definitions (that did appear in the Workprint and are used in the Blade Runner Comic):

android (an'droid) adj. Possessing human features - n.
A synthetic man created from biological materials.
Also called humanoid. (Late Greek androeides,
manlike: ANDR(O) - OID.)

THE AMERICAN HERITAGE
DICTIONARY OF THE ENGLISH
LANGUAGE (1976)

 

android (an'droid) n, Gk. humanoid automaton. more at
robot./ 1. early version utilized for work too
boring, dangerous or unpleasant for humans.
2. second generation bio-engineered. Electronic
relay units and positronic brains. Used in space
to explore inhospitable environments. 3. third
generation synthogenetic. REPLICANT, constructed
of skin/flesh culture. Selected enogenic transfer
conversion. Capable of self perpetuating thought.
paraphysical abilities. Developed for emigration
program.

WEBSTER'S DICTIONARY
New International (2012)

[Note the "positronic brain" in the second generation androids - In ST:TNG, Data has a positronic brain. This particular technology is fictional, BTW; the term was made up by Isaac Asimov as a simple expository term with no scientific background.]

The 1981 script also furnishes us with more detail in the first "Hospital Scene" although dialogue changed before this (deleted) scene was filmed:
HOLDEN: They do a routine autopsy on the one that got aired and.. whaddya know? A skin job, one of the ones that busted out! Top drawer replicant.. combat type.. Nexus six.
DECKARD: Pretty sexy, the sixes.
HOLDEN: Sexy! Three hours into the autopsy they still think they're cutting up a human. No marks, nothing.

Evidently the intention was that these replicants really were almost identical to humans. Their individual abilities in most respects are not "superhuman", but just represent the best of humans. Some humans can withstand extreme cold for a few minutes, walk across hot coals, perhaps even pick an egg out of boiling water, conditioned to withstand a little pain.