A common trope is to have a card that allows you to create mana, and then a way to infinitely bring that card back from the discard pile, allowing for infinite mana. Then you play a card that says something like "spend X mana to deal X damage" and you deal a trillion damage.

[0] the above commenter is correct that a true infinite loop is not allowed. But you're allowed to create a scenario where an infinite loop happens according to a repeating choice you can make (essentially a while loop) and repeat as much as you want within the match timer while still allowing time for your damage card. The general etiquette is just to say "I can do this a billion times and so I deal a billion damage".

I've actually been at a local (mostly informal) tournament where someone with an Elf deck managed to pump their life into the thousands (you start with 20 or 40 depending on the ruleset), and his opponent with a Goblin deck got an infinite combo for infinite damage. But the Elf player refused to accept "I can do this infinity times" and made him actually try to perform his card combo over a thousand times before the round timer ran out. Goblin deck did not win.