If you wish to adopt or purchase a cat, the shelter or seller must first have registered it on CatID via a vet. The change of keeper is then also notified via the vet, at the request of the first keeper. Keepers also have access to the functionality used to send notification of a change of keeper. Each party must accept the change-of-keeper transaction with their ID card via the online application.
If it's a cat you have found, you cannot adopt it directly. For that situation, please read Found cat section.