Environment variables are -by far- the securest AND most practical way to provide configuration and secrets to apps.

Any other way is less secure: files on disk, (cli)arguments, a database, etc. Or about as secure but far more complex and convoluted. I've seen enterprise hosting with a (virtual) mount (nfs, etc) that provides config files - read only - tight permissions, served from a secure vault. A lot of indirection for getting secrets into an app that will still just read them plain text. More secure than env vars? how?

Or some encrypted database/vault that the app can read from using - a shared secret provided as env var or on-disk config file.