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For Celcius I'm less sure, but almost certain that the borrow fees they are collecting from their public-facing retail "Borrow" service in total is not even close enough to cover the APY payouts they're making. I am guessing that they're using a combination of loans to larger accredited investor companies and speculative use and management of the funds, as per an article going into detail about their investment practices:
"But Celsius has also at times invested deposits in perpetual swaps, futures-like contracts with no expiry date, people familiar with Celsius’ business said. "
https://www.coindesk.com/what-crypto-lender-celsius-isnt-telling-its-depositors
As for Gemini, they're very upfront about it:
"Gemini is partnering with accredited third party borrowers including Genesis, who are vetted through our risk management framework which reviews our partners’ collateralization management process. On a periodic basis we will conduct an analysis of our partners’ cash flow, balance sheet, and financial statements to ensure the appropriate risk ratios and healthy financial condition of our partners.
Cryptocurrency, like many assets, can be volatile and subject to price swings. There is always a risk in investing, and each customer needs to assess their own risk tolerance before making any investment decisions. Our partners in Gemini Earn have an obligation to return funds according to the terms of their loan agreement. However, Gemini Earn customers (the lenders) always assume some level of risk when they decide to lend their funds. We believe Gemini Earn gives our retail investors another way to stay long-term in the asset class and have the option to invest and earn interest, all on the Gemini platform."