There is no "perfectly secure method" for storing private key and seed phrases, much like there is no completely risk-free way to store cash or other valuables.
Below are some common storage methods and their respective advantages and disadvantages.
1. Cloud Storage (Google Drive, etc.)
Cloud storage is convenient because it allows you to access your information from different devices. However, there is a risk of your private key or seed phrases being exposed if your cloud account is compromised.
2. Local Device Storage (Smartphones / PCs)
Storing information on your own device allows you to manage it locally. However, there is a risk of losing or exposing your private key or seed phrases due to device failure, loss, or unauthorized access to the device or its stored data.
3. Password Managers
Password managers can store information in an encrypted form, and some can be accessed from multiple devices. However, management methods vary depending on the service or application, so it is important to understand how the service you use handles your information.
4. Paper / Physical Notebooks
Paper storage does not depend on digital devices or online services, but it carries physical risks such as loss, theft, fire, water damage, and deterioration over time.
If you store your information on paper, you may also consider measures such as using a fire- or water-resistant case, or keeping copies in separate locations to account for risks such as fire or water damage.
Keeping a backup in a single location increases the risk of losing access if that location is lost or damaged. On the other hand, storing backups in more places also means considering the additional risks of unauthorized access or theft.
Each method has its own advantages and disadvantages. Consider the balance between the level of security you need and the method you can manage comfortably when choosing how to store your private key or seed phrases.