Hamster Playground

Through normal gameplay by directing your hamsters to do tasks and work, you eventually level up the "house" and can unlock and assign perks so that hamsters do things automatically as needed. After earning and activating some perks, you can then leave the game running passively and continue to accrue XP while not physically playing the game. This is how I initially crossed level 100.

As a side note, I started playing in August 2024, and unlocked every achievement except this one by September 7, ending at house level 126. I crossed 100 during an overnight passive session, but this achievement didn't unlock. It stayed bugged until a recent update, and unlocked on October 15.