This question started as a discussion in the commentaries of What mammalian biological constraints would have to be relaxed/changed in order to allow Victorian style mixing of body parts?
The popular idea is that the monster is made out of pieces of corpses, sewn together, but... two arguments against this:
(1) Quote from ch. 4:
Although I possessed the capacity of bestowing animation, yet to prepare a frame for the reception of it, with all its intricacies of fibres, muscles, and veins, still remained a work of inconceivable difficulty and labour. I doubted at first whether I should attempt the creation of a being like myself, or one of simpler organization; but my imagination was too much exalted by my first success to permit me to doubt of my ability to give life to an animal as complex and wonderful as man.
Sewing parts is equally easy/difficult for humans and animals.
(2) The idea of multiple body pieces is incompatible with the subsequent plot:
Frankenstein destroys the second (female) creature because he don't want a race of monsters. It's really difficult making a sterile creature from pieces of corpses?
EDIT for clarification:
The quote (1) plus the previous research of "... the natural decay and corruption of the human body." suggests some type of guided "taratoembriological" process starting from raw materials simpler than body parts. Some type of non magical Hellraiser-style resurrection.
Relevant ch. 20 quote for (2):
Even if they were to leave Europe and inhabit the deserts of the new world, yet one of the first results of those sympathies for which the daemon thirsted would be children, and a race of devils would be propagated upon the earth who might make the very existence of the species of man a condition precarious and full of terror. Had I right, for my own benefit, to inflict this curse upon everlasting generations?
If Frankenstein don't want this,
making a female monster without an uterus is trivially easy sewing parts, while arguably impossible by "taratoembriological" process.