Sources: 

• The Twilight books (never the movies)
• The Twilight Illustrated Guide
• The Official Site for Twilight (FAQs section)
  

Remember that I do not make the rules of this universe, Meyer did.

Vampire Venom (Turning a Human and Vampire Biology):

• smooths and clears skin --> it could be either nonporous or have microscopic pores--> like marble --> only venom, vampire/werewolf teeth, and vampire/werewolf strength are their weaknesses
• pales and whitens the skin --> racist and self-contradictory in the context of vampire "beauty"(bloodsucking myths and figures exist everywhere in the real world and its peoples’ histories, not just in EU and for EU/white people...plus we're cycling the vampires-be-pale model in media now anyway)
• “crystallizes” human skin (their skin cells, says the official site) to make it reflect direct light --> the look and intensity of the reflectiveness depends on the intensity and the kind of light

“However, the cells that make up their skin are not pliant like our cells, they are hard and reflective like crystal. A fluid similar to the venom in their mouths works as a lubricant between the cells, which makes movement possible.”                                                                          --Meyer, on The Official Site

• “crystallizes” human skin and flesh to make it very cold--> like “ice”, as Bella-the-human describes
• “crystallizes” and hardens the former human's tissues so that when they transform they are practically invulnerable --> includes the eyeballs
• makes the body able to withstand all physical pressures (except vampire/werewolf teeth) -->which explains how it is their strength and physical resilience that allows them to go very fast: their hard bodies can take a lot of physical shocks, as I assume Meyer would explain it
• gives super strength --> humans have absolutely no chance of hurting them no matter how strong they are --> the biggest or strongest human is always weaker than the smallest “weaker” vampire. Vampire Alice would beat human Emmett hands down.
• gives super speed -->  humans can’t detect/keep track of their natural movements as they occur
• *straightens out the more superficial asymmetries of the face -->lending them their preternatural beauty (debatable on the very concept of "perfection")
• removes and stops all organic cycles  (except for venom-based-lubricants for movements, sperm production[?] and "nutrient” absorption) --> physical changelessness --> they can’t get thinner or thicker or physically mature
• sharpens the former human's senses (vision, hearing, etc.), reflexes, & instincts --> so their reflexes literally prevent them so tripping or dropping stuff by accident --> to the point where vampires act based on their instincts and how much control over them, to various degrees --> vampires can see and hear from both longer distances & spot things humans will never be able to see up close --> their sense of smell is their dominant faculty-- the smell of blood makes vampires' throats burn similar to how a cooked chicken makes my stomach rumble....but much worse as if I was starving rather than just hungry --is why they find it so hard to control themselves when they smell blood, especially exposed to the air
• renders the new vampire incapable of being unalert to their environment-->so they also do not sleep
  
• replaces former human hunger sensation with the sensation of "burning" that intensely compels vampires to attack drink from the nearest blood-filled thing --> some vampires describe it as “fire”, others as “acid” or extreme “dryness” --> gets worse the longer vampires don’t feed
  
• freezes the person in whatever psychological/mental state they had before they were turned --> it seems that Meyer meant both the brain’s physical maturity and the person’s personality (determined by ages and circumstances-when-turned)
• replaces blood and other fluids with venom-based substances --> including the blood that is used to raise the penis during arousal (but sperm is still sperm)
• is highly flammable-->makes the new vampire able to be killed through fire, but only after tearing their bodies apart so that they can't run fast enough to put out the fire 
• vampire venom is poisonous and lethal to Quileute "werewolves", aka shapeshifters (my mod will never have Q. werewolves) 

The Children of the Moon, a.k.a, The "Traditional" Werewolves in Twilight:

(If I ever make my mod include werewolves...I'm never making Quileute werewolves. These will be the werewolves I'd make.) 

• they change by the full moon only and when they change they look half-human, half-wolf
• they cannot control any of their urges when they are in their furry state
• they are as strong and fast as vampires
• they have been hunted down to near extinction by the Volturi because of their strength, so they are extremely rare
• they have more powerful forelegs than hind legs, making them more ape-like than canine (think Remus Lupin)
• they are immune to vampire venom (both the pain-inducement and the transformative element)
• they reproduce themselves through a bite like vampires do
• they can't reproduce sexually, so they don't pass down their state through sex or genetics
• they travel either alone or with one other werewolf that is of a different sex than themselves

Other Stuff to Know:

• vampires with female reproductive systems cannot give birth because of the reasons above concerning the hardness and intractability of flesh (the canonical reason)
• vampires with male repro. systems can impregnate female humans --> the offspring are known as hybrid vampires...e.g., Renesmee Cullen
• vampires can have many different romantic and/or sexual partners in their existence---HOWEVER, if they ever happen to devote themselves to one person, that person will be the one whom the average vampire will never truly get over, especially if they die --> vampires call their devoted lover/partner their "mate".....e.g., Rosalie and Emmett are mates; another mated pair are James and Victoria --> (the jury is out for polyamorous triads, quads, etc. or Vs...none of these appear in the novels and thus do not seem canonical)
• as of April 2, 2022, there is no canonical evidence of magic/witchery in the Twilight universe; HOWEVER, given that magic can be argued as just the psyche interacting with the material world and conducted through focus and education, we can say that with enough imagination, it would be very easy to insert witchcraft into the Twilight universe...however, to me, it still  probably will be nothing like Lord of the Rings

Comment Below with your Thoughts! (but not about my mod, only about Twilight Lore, Fandom, etc.)