Your bitcoin is still yours, and you can still get to it. It sits in its own vault on the bitcoin network, titled in your name, and it isn't an asset on Onramp's balance sheet.
Three institutions protect it. If Onramp stopped operating, the other two, BitGo and CoinCover, can carry out a documented recovery process with you without Onramp. You can download your recovery instructions anytime: click your name in the top right corner of the dashboard, select Settings, and under Recovery, click Download PDF Instructions.