confusion about rebase token steth

Lido DAOLido DAO
self.LidoFinance1m ago
If I buy some steth from dex, but I dont stake eth, why I can still get rewards by rebasing. For example, the pool have 100 eth stake on beacon chain deposited by userA, and tomorrow 10 eth is rewards, then 10 steth will be mint and sent to userA, but if I buy 10steth from dex, the day after tomorrow, 10 eth rewards again, I can get some rewards on proportion of all steth holders, I will get 0.1 or 0.2 out of this 10 eth rewards base on proportion in the form of steth, so why I can get the rewards as actually userA is the person stake the eth on beacon chain?