I have a rule as DM that I will never fudge rolls against the players, but I will fudge rolls in their favor if it fits the narrative. Three players consecutively miss an enemy? Oh no, next turn it got a critical fail and then failed a Dex save, slipping and landing prone. I guess the players get advantage on melee attacks! Don't do this often, but I'm the right spot it's fine.

Monsters can just be dumb, too. INT of 6 means those Blobby Blobs are gonna fight poorly, attacking the tank and splitting up attacks. Also, the players don't know your monsters' stats, so you can make an attack that would get it to 1 HP instead kill an enemy. I also don't do this often, but sometimes they just get unlucky, and no reason to TPK.

Speaking of TPK, you can let them fail forward. Monsters rarely have reason to attack downed/unconscious PCs, so let them roll death saves, but it usually takes a while to die. Everyone's unconscious? Stop combat, no more death saves. Instead, they wake up as prisoners and have to escape. I had a particularly fun one where my players would have died to a trio of night hags (if you've played Curse of Strahd, you know the ones). Instead, they woke up and were given a mission, but the bags also stole parts of their "souls," taking Max HP from one player, speed from another, giving one a trait that they would lose all hope, stealing another's eye (disadvantage on Perception), etc. Then they had to fulfill the mission, but later came back at a higher level and beat the hags, gaining their souls back. Everyone loved getting revenge!