Atomic wallet is a closed source wallet, just like the popular trust wallet and others, they are not recommended because they are closed source and it is impossible to verify their code. Wallets like Electrum is open source and well reviewed, so everything is open and can be verified, it is a software wallet, but safer than Atomic wallet and trust wallet.
However, if you are going to store a large amount in BTC, you should run your Electrum wallet in an airgapped device that will be completely disconnected from the internet, because anything online is prone to hacking.