https://en.uesp.net/wiki/Online:CompanionsCompanions level up via a percentage (15% + Rapport Bonus), rounded up, of non-Enlightenment experience gained with your character directly from combat.
Experience increasing effects such as holiday bonuses and consumable boosters applied to your character will provide an indirect benefit to the Companion as well.
Nach dem Lesen eurer Beiträge und Nachschlagen auf UESP verstehe ich das Ganze so:Each rapport tier above Cordial provides an additional 5% absolute experience gain.