None of the Cele Trei Creaturi protags are actually humans they are all dirty faking shapeshifters.
The three Vampiric Species are all built to shapeshift from the cellular level up. Their genome is divided into three distinct sections for each of the three forms they can take (Original Form, Titanic Form, and Human Form) so they can function indefinately in any of them. In-between forms are rather unstable and tend to require concentration to maintain. Transformation itself is very quick, taking about 2-3 seconds on average, and does involve the actual disassembling and creation of matter from ambient magical energy (matter in CTC can be converted into energy and vice versa) depending on the weight of the form.
The magical abilities of the Vampiric Species are limited to their perception of their body, so clothes transform with them unless they are actively thinking about how their clothes are not part of them (and who does that). A talented Vampire can even get small backpacks or objects to transform with them through mental self-trickery. The process by which the clothes and objects transform into the body is called storing.