They don't. As per the PHB page 194 this is what is said about 1 and 20:
ROLLING I OR 20
Sometimes fate blesses ar curses a combatant, causing
the novice to hit and the veteran to miss.
If the d20 rol1for an attack is a 20, the attack hits
regardless of any modifiers ar the target's AC. In
addition, the attack is a criticai hit, as explained later
in this chapter.
If the d20 roll for an attack is a I, the attack misses
regardless of any modifiers ar the target's AC.
That's for attack rolls. no equivalent can be found in the pages covering ability checks or saving throws.
