The Kongolese are receiving the theming bonuses from three Archaeological Museums and three Art Museums.
A theming bonus is a 
Culture and 
Tourism bonus received by filling Museums with Great Works of Art or 
Artifacts that satisfy certain criteria. When activated, the bonus doubles the yields of all items in the Museum.
When playing as Kongo, the 
Food, 
Production, and 
Gold bonuses also get doubled if you manage to theme an Art Museum with 
Sculptures or an Archaeological Museum with Artifacts. Theming also doubles the 
Science bonus to Great Works granted by the Anshan city-state.
Archaeological Museum[]
To receive the theming bonus from an Archaeological Museum, its slots must be filled with Artifacts from the same era and different civilizations. Each city-state and Free City counts as a distinct civilization for this purpose, as do the barbarians.
In vanilla Civilization VI and Rise and Fall, England's civilization ability automatically themes their Archaeological Museums once all their slots have been filled.
Art Museum[]
To receive the theming bonus from an Art Museum, its slots must be filled with Great Works of Art of the same type (i.e. Sculptures, 
Portraits, 
Landscapes, or 
Religious) made by different Great Artists. This means that a minimum of three Great Artists are needed to activate each Art Museum's theming bonus.
Kristina[]
Kristina is receiving massive theming bonuses because of her unique ability and Reliquaries belief. Screenshot taken from here.
In Gathering Storm, Kristina's leader ability automatically themes all her buildings with at least three Great Work slots and all her wonders with at least two Great Work slots once all their slots have been filled.
