I'll break down how I understand this row, found in the default Leaders.xml
Code:
<Row>
<ModifierId>HIGH_DIFFICULTY_COMBAT_SCALING</ModifierId>
<Name>Amount</Name>
<Type>LinearScaleFromDefaultHandicap</Type>
<Value>-1</Value>
<Extra>1</Extra>
</Row>
The default handicap is always prince. The Value field gives that much strength to every difficulty where this modifier has met its requirement (prince or higher). The Extra field gives +1 strength to prince, but counts +1 more strength on each successive difficulty as well (+2 to king, +3 to emperor, etc). Value and Extra are added together to get the final result, so at prince you get 1(extra) -1(value) = +0 strength, at king you get 2(extra) -1(value) = +1 strength, etc.
So for your changes:
If I would set Value to -2 instead of -1, then the AI would only have a +2 combat strength instead of the normal vanilla +3 combat strength on the difficulty Immortal?
This would affect every difficulty prince and higher. Prince would have -1 strength, king would have +0, emperor +1, etc.
If I would set Extra to 0, then the AI would not scale any combat strength at all after the difficulty Prince?
Correct, although because value is still -1, all difficulty levels prince or higher would have -1 strength.
If I would set Extra to 2, then the AI would have a +4 combat strength on the difficulty Immortal?
This would result in prince having 2(extra) -1(value) = +1 strength, king having 4(extra) -1(value) = +3 strength, etc going up by 2 strength on each difficulty level.
The reason this is so complicated is that Firaxis wanted a function that can set the combat difficulty (and other bonuses) linearly with a single modifier, in that pattern of adding the same amount more on each difficulty level. But that means it kinda becomes a nightmare if you want to set different difficulty levels to exact amounts.