Open source, non-custodial client for BTC, ETH and ERC20-USDC

I’m new to Ethereum and looking for an open source, non-custodial client. I’m not sure if I use the term “non-custodial” correctly though. In essence, I want to have two apps: one initiates transactions on the wallet, the other signs them.

I tried to look for such client on ethereum.org, but found only AirGap. Other products provide just one app, e.g. Coin Wallet. I don’t understand. One app must store private keys, the other must have no access to them.

Coin Wallet doesn’t accept the passphrase of a wallet I’ve made earlier for BitCoin.

I’ve also noticed that AirGap Vault doesn’t support ERC20-USDC. Why? Who does?

What am I missing? What is the best way to achieve my workflow?