Newbie needs help

I bought some ETH on ftx.us. I wanted to stake, but I couldn't do so on ftx.us. So I transferred the ETH to the FTX android app on my phone by providing the Ethereum address displayed on the FTX app. My ETH is no longer on ftx.us, but is on the FTX app on my phone, and I appear to be earning ETH through staking now.

My question is where exactly does my ETH reside now? It is on my phone, or on another FTX exchange? Why is ftx.us different from the FTX android app?

Can I stake if my ETH were on a hardware wallet such as ledger?