Pharmacokinetics is the study of what the body does to a medicine: how it is absorbed, spread through the body, broken down and removed.
Pharmacodynamics is the opposite question, which is what the medicine does to the body. Together they explain why the same dose can feel different for different people.
For ADHD medication, pharmacokinetics explains when a dose starts working, when it peaks and when it wears off. Long-acting medicines are designed to change that timing so one morning dose lasts most of the day. Body size, metabolism, food and other medicines can all change the timing too.
That is why titration takes a few weeks. Your psychiatrist adjusts the dose and timing based on what you notice, rather than on a fixed formula.