Exchanges in this context are centralized exchanges that only provide custodial wallets for their users and we know "Not your keys, not your coins". People only can have their keys if they use non custodial wallets and be own custodian of their bitcoin. Otherwise, they can use custodial wallets from centralized exchanges but they don't have access to private keys, and they don't actually own any bitcoin.

Secondly, do not ever consider storing your seed phrase on your phone.
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Your phone and other mobile devices are hot wallets, nothing more than hot wallets. You should never store all fund on a phone or most of your fund on any mobile device as it is very dangerous practice.