In Plants vs Brainrots, when you rebirth, you do lose all your collected cash, but you do NOT lose your brainrots. Brainrots, along with your plants, gear, and eggs, remain intact after rebirthing. However, some sources mention that earlier rebirthing could cause loss of brainrots, so many players recommend gifting or storing your precious brainrots with friends or on alternate accounts before rebirthing as a precaution. Overall, the main things lost on rebirth are cash, boss levels, and spawner upgrades, but your brainrots stay with you.