Modifications work on scores rather than modifiers.
When character gains an ability score improvement, usually at levels 4, 8, 12, 16, and 19 (though the fighter and rogue get more) the characters' ability scores are increased, rather than their modifiers.
The same goes for magic items that increase or modify ability scores, such as the tome of understanding (and other tomes), the manual of bodily health and the star card from the deck of many things, as well as items like gauntlets of ogre power increase scores instead of modifiers.
Reductions in abilities also works on scores: see strength drain of the shadow and devour intellect of the intellect devourer.
In the same way, scores are determined at character generation, rather than modifiers.
The strength score is additionally used for the calculation of carrying capacity and as prerequisite for some armor types.
Of course the scores will determine the modifiers as well. But modifiers are not changed directly.