Why does jamais come afetr the verb?

Bonjour Taejah, That is a fixed form that is used in negation. Similarly there are other fixed forms in French negation such as ne. . . pas, ne. . .plus, etc. Please see the following link which goes into greater detail. https://progress.lawlessfrench.com/my-languages/french/glossary/24 To that end, in compound verb tenses and moods, «ne precedes the auxiliary verb with «jamais», for example, following it, «jamais» is then followed by the past participle. Example: Je n'ai jamais voulu manger encore de pomme. Bonne chance et bonne continuation.