Looks like it might be the Leaders.XML file.
These appear to be the lines specifically relating to the bonuses I'm after:
Code:
<Row>
<ModifierId>HIGH_DIFFICULTY_SCIENCE_SCALING</ModifierId>
<Name>Amount</Name>
<Type>LinearScaleFromDefaultHandicap</Type>
<Value>0</Value>
<Extra>8</Extra>
</Row>
<Row>
<ModifierId>HIGH_DIFFICULTY_SCIENCE_SCALING</ModifierId>
<Name>YieldType</Name>
<Value>YIELD_SCIENCE</Value>
</Row>
<Row>
<ModifierId>HIGH_DIFFICULTY_CULTURE_SCALING</ModifierId>
<Name>Amount</Name>
<Type>LinearScaleFromDefaultHandicap</Type>
<Value>0</Value>
<Extra>8</Extra>
</Row>
<Row>
<ModifierId>HIGH_DIFFICULTY_CULTURE_SCALING</ModifierId>
<Name>YieldType</Name>
<Value>YIELD_CULTURE</Value>
</Row>
<Row>
<ModifierId>HIGH_DIFFICULTY_PRODUCTION_SCALING</ModifierId>
<Name>Amount</Name>
<Type>LinearScaleFromDefaultHandicap</Type>
<Value>0</Value>
<Extra>20</Extra>
</Row>
<Row>
<ModifierId>HIGH_DIFFICULTY_PRODUCTION_SCALING</ModifierId>
<Name>YieldType</Name>
<Value>YIELD_PRODUCTION</Value>
</Row>
<Row>
<ModifierId>HIGH_DIFFICULTY_GOLD_SCALING</ModifierId>
<Name>Amount</Name>
<Type>LinearScaleFromDefaultHandicap</Type>
<Value>0</Value>
<Extra>20</Extra>
</Row>
<Row>
<ModifierId>HIGH_DIFFICULTY_GOLD_SCALING</ModifierId>
<Name>YieldType</Name>
<Value>YIELD_GOLD</Value>
</Row>
<Row>
<ModifierId>HIGH_DIFFICULTY_FAITH_SCALING</ModifierId>
<Name>Amount</Name>
<Type>LinearScaleFromDefaultHandicap</Type>
<Value>0</Value>
<Extra>8</Extra>
</Row>
<Row>
<ModifierId>HIGH_DIFFICULTY_FAITH_SCALING</ModifierId>
<Name>YieldType</Name>
<Value>YIELD_FAITH</Value>
</Row>
I'm not certain, but my reading is that this means that at Default difficulty of Prince, the values are 0, and each difficulty level above increases it in scalar fashion by the amount specified.
Science: 8
Culture: 8
Production: 20
Gold: 20
Faith: 8
This would correlate with the table from the Wiki, which I've reproduced below:
Source: https://civilization.fandom.com/wiki/Difficulty_level_(Civ6)
The Wiki table shows these numbers as representing percentage points, although the XML file doesn't mention this. Will assume Wiki is correct on this.
So if I wanted the AI bonuses to be between Emperor and Immortal, I would want AI Science/Culture/
Faith bonuses at + 20 (%), and Production/Gold at + 50 (%). To affect this change, I would presumably change the <Extra></Extra> values of Science/Culture/Gold from 8 to 10, and the Production/Gold <Extra></Extra> values from 20 to 25.
Apart from the free starting Techs/Civics and units, what's left are the AI Combat and XP bonuses. The AI Combat bonus goes up by 1 each difficulty level. So, between Emperor and Immortal, it would be + 2.5. That would mean a scale value of 1.25, rather than 1. Not sure if the game could handle non-whole numbers, but I could test it out.