Originally when I started playing, you just looked up an Attack Matrix. This listed the number to hit an Armor Class based on the character class and level. Armor Class was Descending, so lower was better. Once you knew one AC, of course, a simple calculation gets you the rest. You need a 19 to hit Armor Class 0 then obviously a 17 to hit Armor Class 2. Moldvay Basic or AD&D.
This of course led to the second edition and recording the To Hit Armor Class 0 number or THACO on a character sheet.
Another way to calculate this, which gets used in Stars Without Number 1st Edition for example:
d20 + modifiers + AC >= 20
Here you have at a simple level, an attack bonus for example. If this is +2 and the AC of the opponent is 5 then you need d20 + 7 >= 20 or a 13 to hit. If instead of wearing Chain the opponent is wearing Plate, then it would be a 15 to hit.
Then we have a current version of Ascending Armor Class where the Armor Class listed is the target number to hit.
d20 + modifiers >= AC
