If you choose your wallet or phone software, you need to be extra careful! Remember that bitcoins are stored inside your device. If your computer or phone breaks down, your bitcoin is gone too!
Bitcoins are not stored inside a device. The private keys, which are necessary to use/move the corresponding UTXO's (which have been confirmed on the blockchain), are stored inside your device.
A simple backup of your private keys (or better: seed) can counteract a loss of bitcoins if the device breaks.
Do you guys have a better idea to avoid theft in the wallet?
Split your holdings. Majority in a secured cold wallet and a small stack on the mobile/desktop wallet for daily purchases.
Or: Use a hardware wallet with seed being backed up in multiple secured places.
The key is to reduce the possible attack surface to a minimum.
And use your common sense. BTC's not only get stolen through digial/physical theft, but also through shitty ICO projects.
Yeah the biggest thing here is not putting everything in the same account. I heard a story of a guy who lost his harddrive once and there was nearly a million worth of bitcoin on there. It got mistakenly thrown in the dump and he chased after it, went riffling through the landfill for a week but still couldn't find it.
Just keep the main copy on an external drive and consider that your bank account. Make a copy on another external drive. Write down the seed somewhere safe.