Core rule
Natural die results and totals
The number on a die and the final total after bonuses are different.
For attack rolls, a natural 1 misses and a natural 20 hits regardless of Armour Class, normally producing a critical hit. Rolling 15 and adding +5 is not a natural 20.
Other effects can expand the critical range or enable critical hits under special conditions. Read those effects separately, and do not apply attack rules to every other check.