There are many different types of conlangs. Auxiliary languages (or auxlangs) are designed to facilitate international communication. They are called auxiliary languages because they are designed to be learned along with other languages. Their aim is to provide a common language to people who don’t have any other language in common.
Naturalistic conlangs on the other hand, are designed to emulate how natural languages work. Dothraki, High Valyrian, Klingon, Na’Vi and my own conlang Atlaans are examples of naturalistic conlangs. Natural languages tend to have irregular conjugation, multiple declensions and inconsistent orthography due to changes in the language that happen over time.
Naturalistic conlangs either come up with their own conjugations, or they come up with some older languages and evolve it in like with linguistics processes to its modern form. This is what David J. Peterson did for both Dothraki and High Valyrian. He came up with an older form, and then created the sound shifts and semantic changes that happened over time to create the modern version of the language