You can earn interest on Bitcoin using trustless and decentralized processes.
Here are the steps that you can use to do that using NebliDex, an open sourced atomic swap exchange:
https://www.neblidex.xyz/1) Grab the NebliDex application, there is a mobile client available. The source code is available too so you can build from source and run if you prefer to do so.
2) Deposit Bitcoin into your trading wallet by sending BTC to your client wallet. Because this exchange is decentralized, all coin information, including the private key, is stored locally on your phone. There is no exchange wallet. Make sure you obtain sufficient NebliDex tokens (NDEX) as well because these tokens are used to pay trading fees to the exchange relayers known as Critical Nodes. You can obtain these tokens through the exchange. You will also need a bit of ETH to interact with ETH smart contracts.
3) Go to the WBTC/BTC market and exchange your BTC for wBTC (which is an ETH token that is pegged 1:1 with Bitcoin).
4) Withdraw your wBTC and send it to one of various decentralized DeFi solutions (Compound.finance or Fulcrum.trade)
5) Earn passive interest and withdraw when ready. Repeat these steps in reverse to go back to Bitcoin. All steps can be accomplished without identity checks or signing up.
For more information on what WBTC is, go here:
https://www.wbtc.network/.