Electrum wallet is open source wallet and most secure wallet so far. you can use it in the mobile (Android+Ios) and desktop.
Electrum does not support iOS. I will recommend Bluewallet for iOS.
If you wanting only to store btc then it is best choice but if you want to store multiple coins like Btc, Ethereum and other in one place then I will recommend Trustwallet (10 m download) or Safepal wallet (1m download).
Trustwallwt is close source wallet. Safepal is still a close source wallet for now. I will not recommend both. I will recommend open source wallet like Unstoppable wallet, or hardware wallet like Trezor.